How to find CRC byte of a image file

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I have a image file named "xyz.img". I need to change some of the content in the image file (say one or two bytes). I hexdump it by using the command

hexdump -x xyz.img > hex1

Now when i am opening the "hex1" using vi, it is showing like this:

0000000    ffff    ffff    ffff    ffff    ffff    ffff    ffff    ffff
*
0000020    ffff    ffff    00ff    ffff    ffff    ffff    ffff    ffff
0000030    ffff    ffff    ffff    ffff    ffff    ffff    ffff    ffff
*
0001000    0568    28ca    b438    0420    f746    1080    ffff    ffff
0001010    e469    8108    026b    a01f    8086    10d3    ffff    9c5a
0001020    0000    2001    7e94    ffff    1000    0048    0000    2704
0001030    6cc9    3150    073e    460b    2d84    0140    f000    0706
0001040    6000    7100    1408    ffff    4d01    92ec    fc5c    f083
0001050    0028    0233    0050    7d1f    1961    0453    00a0    ffff
0001060    0100    4000    1315    4003    ffff    ffff    ffff    ffff
0001070    ffff    ffff    ffff    ff..................................
.......................................................................
..........................................................

I couldn't post the whole as it is too large. Now I need to replace one or two bytes. Ultimately my CRC Byte will also get change.

where this CRC is located and how could I modify the CRC as per as the replaced bytes?

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