how to replace the values of a key in a dictionary?

You don't need to iterate to change the dictionary.
Try

dict.Remove(keyToUpdate);
dict.Add(newKey, value);

You will not be able to change the collection while you are iterating through it.
A way out is to create a temporary list of key value pairs, iterate through this list and change the key as appropriate.

See this post to help you,http://stackoverflow.com/questions/6499334/best-way-to-change-dictionary-key[^], you can't rename keys once that have been added to a Dictionary, first remove then add a new.