How to update std::map after using the find method?

std::map::find returns an iterator to the found element (or to the end() if the element was not found). So long as the map is not const, you can modify the element pointed to by the iterator:

std::map<char, int> m;
m.insert(std::make_pair('c', 0));  // c is for cookie

std::map<char, int>::iterator it = m.find('c'); 
if (it != m.end())
    it->second = 42;

I would use the operator[].

map <char, int> m1;

m1['G'] ++;  // If the element 'G' does not exist then it is created and 
             // initialized to zero. A reference to the internal value
             // is returned. so that the ++ operator can be applied.

// If 'G' did not exist it now exist and is 1.
// If 'G' had a value of 'n' it now has a value of 'n+1'

So using this technique it becomes really easy to read all the character from a stream and count them:

map <char, int>                m1;
std::ifstream                  file("Plop");
std::istreambuf_iterator<char> end;

for(std::istreambuf_iterator<char> loop(file); loop != end; ++loop)
{
    ++m1[*loop]; // prefer prefix increment out of habbit
}

You can use std::map::at member function, it returns a reference to the mapped value of the element identified with key k.

std::map<char,int> mymap = {
                               { 'a', 0 },
                               { 'b', 0 },
                           };

  mymap.at('a') = 10;
  mymap.at('b') = 20;