In unordered_map of C++11, how to update the value of a particular key?

If you know that the key is in the map, you can utilize operator[] which returns a reference to the mapped value. Hence it will be map[key] = new_value. Be careful, however, as this will insert a (key, new_value) if the key does not already exist in the map.

You can also use find which returns an iterator to the value:

auto it = map.find(key)
if(it != map.end()) 
    it->second = new_value;