How to increase value on key element in map?

Question

I have a map in c++ with following pairs:

pair<int, int>(1, 40) 
pair<int, int>(2, 30) 
pair<int, int>(3, 60) 
pair<int, int>(4, 20) 
pair<int, int>(5, 50) 
pair<int, int>(6, 50) 
pair<int, int>(7, 10) 

How can I increase pair with key "1" by 2? so how can I get this:

pair<int, int>(1, 42) 
pair<int, int>(2, 30) 
pair<int, int>(3, 60) 
pair<int, int>(4, 20) 
pair<int, int>(5, 50) 
pair<int, int>(6, 50) 
pair<int, int>(7, 10) 
```