How to update second value of map using iterator

I have a function in which it checks for the characters and number of times it is repeated in a string.

It is stored as (for example)string is "hello" [h]=>1 [e]=>1 [l]=>2 [o]=1

Whenever a letter occurs more than once I need to update it.

I tried using

it->second = it->second+1;

But it doesn't works How can I do that?

Full code is

int fn(string a) {
  map<char,int> mymap;
   for(int i=0;i<a.size();i++)
   {
       std::map<char, int>::iterator it = mymap.find(i);
       if(it!=mymap.end())
       {
            //say i need to update occurrence from 1 to 2 or 2 to 3...
           it->second = it->second+1;//(how can i do that)
       }
       else
       mymap.insert(pair<char,int>(a[i],1));
   }
   std::map<char,int>::iterator i;
   for(i=mymap.begin();i!=mymap.end();i++)
   {
       cout<<i->first<<i->second;
   }
}