By mistake i entered wrong college name and now I want to change it? What is the optimal way to do it or it is NP problem ?
You can do it in constant time, by going to your profile and clicking on EDIT PROFILE.
You can do it in constant time as bugkiller has mentioned but still it is a NP problem. Check out the wiki article http://en.wikipedia.org/wiki/NP_(complexity) an especially the attached figure in it.
Maybe you meant is it an NP-Complete problem then the answer is No.