How do I give my cell a value?

Question

I am making a table in html where I have to calculate a price after discount. I have for the moment 2 rows with 7 header cells and under each header cell I have a cell. I am struggling to add values to my cells. I tried this var cells = document.getElementById('table').getElementsByTagName('td'); cells[1] = "3"; to add the value 3 to my Quantity but the value does not appear on my table. I want to do it this way so I can later apply the convert function to all my Tds. What am I doing wrong?

/* Format amount */
function ConvertAmount(Amount) {
  var cells = document.getElementById('table').getElementsByTagName('td');
  cells[1] = "3";

  var Zahl = Amount;
  Zahl = Math.round(Zahl * Math.pow(10, 2)) / Math.pow(10, 2);
  Zahl = Zahl.toFixed(2);
  Zahl = Zahl.toString();

  var Negativ = false;
  if (Zahl.indexOf("-") == 0) {
    Negativ = true;
    Zahl = Zahl.replace("-", "");
  }

  var str = Zahl.toString();
  str = str.replace(".", ",");

 
  var intVorkommastellen = str.length - (str.length - str.indexOf(","));

  var intKTrenner = Math.floor((intVorkommastellen - 1) / 3);
 
  var intZiffern = (intVorkommastellen % 3 == 0) ? 3 : (intVorkommastellen % 3);

 
  strNew = str.substring(0, intZiffern);
  
  strHelp = str.substr(intZiffern, (str.length - intZiffern));

  
  for (var i = 0; i < intKTrenner; i++) {
    
    strNew += "." + strHelp.substring(0, 3);
   
    strHelp = strHelp.substr(intZiffern, (strHelp.length - intZiffern));
  }

 
  var szNachkommastellen = str.substring(intVorkommastellen, str.length);
  if (szNachkommastellen.length < 3) {
    strNew += str.substring(intVorkommastellen, str.length) + '0';
  } else {
    strNew += str.substring(intVorkommastellen, str.length);
  }
  var Zahl = strNew;

  if (Negativ) {
    Zahl = "- " + Zahl;
  }

  return Zahl;
}

.briefkopf {
  margin-top: 5px;
}
.ueberschrift {
  font-size: 20pt;
  font-weight: bolder;
}
.unterueberschrift {
  font-size: 14pt;
}
.dagufarbe {
  color: #EC7404;
}
body {
  margin: 0;
  padding: 0;
  font: 12pt"Arial";
}
.page {
  width: 21cm;
  min-height: 29.7cm;
  padding: 1cm;
  margin-left: 50px;
}
.subpage {
  padding: 1cm;
  border: 5px red solid;
  height: 256mm;
  outline: 2cm #FFEAEA solid;
  page-break-before: always;
  page-break-after: always;
}
@page {
  size: A4;
  margin: 0;
}
@media print {
  .page {
    margin: 0;
    border: initial;
    border-radius: initial;
    width: initial;
    min-height: initial;
    box-shadow: initial;
    background: initial;
    page-break-after: always;
  }
}

<div class="page">
  <div class="briefkopf">
    <br>
    <br>
    <br>
    <div>
      <script type="text/javascript">
      </script>
    </div>
    <div>street</div>
    <div>number</div>
    <div>country</div>
  </div>
  <br>
  <br>
  <div class="ueberschrift">Offer from
    <script type="text/javascript">
      document.write(Invoicedate)
    </script>
  </div>
  <br>

  <style type="text/css">
    .tg {
      border-collapse: collapse;
      border-spacing: 0;
    }
    .tg td {
      font-family: Arial, sans-serif;
      font-size: 12pt;
      padding: 10px 5px;
      border-width: 1px;
      border-style: none none solid;
    }
    .tg th {
      font-family: Arial, sans-serif;
      font-size: 12pt;
      font-weight: normal;
      padding: 10px 5px;
      border-width: 3px;
      border-style: none none solid;
    }
  </style>
  <table id="table" class="tg" style="width:900px;">
    <tr>
      <th align="center" class="tg">Pos</th>
      <th id="Quantity" align="center" class="tg">Quantity</th>
      <th align="center" class="tg">Type</th>
      <th align="left" class="tg">Description</th>
      <th align="center" class="tg" style="width:100px;">Unitprice</th>
      <th align="center" class="tg" style="width:75px;">Discount</th>
      <th align="center" class="tg">Totalprice</th>
    </tr>

    <tr style="page-break-inside:avoid;">
      <td align="right" class="tg" valign="top">
        <center>
          <script type="text/javascript">
            document.write(ConvertAmount('{{POSITION}}'))
          </script>
        </center>
      </td>
      <td align="right" class="tg" valign="top">
        <center>
          <script type="text/javascript">
            document.write(ConvertAmount('{{QUANTITY}}'))
          </script>
        </center>
      </td>
      <td align="right" class="tg" valign="top">
        <center>
          <script type="text/javascript">
            document.write(ConvertAmount('{{ART}}'))
          </script>
        </center>
      </td>
      <td align="left" class="tg">
        <div class="dagufarbe">
          <center>
            <script type="text/javascript">
              document.write(ConvertAmount('{{DESCRIPTION}}'))
            </script>
          </center>
        </div>
        <br>
        <div style="margin-top:-15px;">
          <script type="text/javascript">
            document.write(ConvertAmount('{{COMMENTARY}}'))
          </script>
        </div>
      </td>
      <td align="right" class="tg" valign="top">
        <script type="text/javascript">
          document.write(ConvertAmount('{{UNITPRICE}}'))
        </script>€</td>
      <td align="right" class="tg" valign="top">
        <script type="text/javascript">
          document.write(ConvertAmount('{{DISCOUNT}}'))
        </script>%</td>
      <td align="right" class="tg" valign="top">
        <script type="text/javascript">
          document.write(ConvertAmount('{{POSITIONSPRICE}}'))
        </script>€</td>
    </tr>
  </table>
  <style type="text/css">
    .ta {
      font-family: Arial, sans-serif;
      font-size: 12pt;
      padding: 10px 5px;
      overflow: hidden;
      word-break: normal;
    }
    .tb {
      font-family: Arial, sans-serif;
      font-size: 12pt;
      font-weight: normal;
      padding: 10px 5px;
      border-width: 3px;
      border-style: none none solid;
    }
    .te {
      font-family: Arial, sans-serif;
      font-size: 12pt;
      font-weight: normal;
      padding: 10px 5px;
      border-style: none none solid;
    }
  </style>
  <table style="width:575px; margin-left:330px;">
    <tr>
      <td class="ta">Total nett
        <div style="margin-left:500px; margin-top:-20px;">
          <script type="text/javascript">
            document.write(ConvertAmount('{{AMOUNTNETT}}'))
          </script>€</div>
        <br>
      </td>
    </tr>
    <tr>
      <td class="te">
        <div style="margin-top:-25px;">plus VAT 19,00 %</div>
        <div style="margin-left:500px; margin-top:-20px;">
          <script type="text/javascript">
            document.write(ConvertAmount('{{AMOUNTTAX}}'))
          </script>€</div>
        <br>
      </td>
    </tr>
    <tr>
      <td class="tb"><b>Total amount <div style="margin-left:500px; margin-top:-20px;"><script type="text/javascript">document.write(ConvertAmount('{{AMOUNTGROSS}}'))</script> €</div></b>
      </td>
    </tr>
  </table>
  <br>
</div>
<br>


</p>