modify a specified commit in git

First rebase to the commit right before the one you want to change. Make the change. Commit. Rebase back.

http://stackoverflow.com/questions/1186535/how-to-modify-a-specified-commit 

You can use git rebase, for example, if you want to modify back to commit bbc643cd, run

$ git rebase --interactive bbc643cd^ 

In the default editor, modify ‘pick’ to ‘edit’ in the line whose commit you want to modify. Make your changes and then commit them with the same message you had before:

$ git commit -a --amend --no-edit 

to modify the commit, and after that

$ git rebase --continue 

to return back to the previous head commit.