update: added safer method

preferred method:

  1. check out the previous (unchanged) state of your file; notice the double dash

    git checkout HEAD^ -- /path/to/file
  2. commit it:

    git commit -am "revert changes on this file, not finished with it yet"
  3. push it, no force needed:

    git push
  4. get back to your unfinished work, again do (3 times arrow up):

    git checkout HEAD^ -- /path/to/file

effectively 'uncommitting':

To modify the last commit of the repository HEAD, obfuscating your accidentally pushed work, while potentially running into a conflict with your colleague who may have pulled it already, and who will grow grey hair and lose lots of time trying to reconcile his local branch head with the central one:

To remove file change from last commit:

  1. to revert the file to the state before the last commit, do:

    git checkout HEAD^ /path/to/file
  2. to update the last commit with the reverted file, do:

    git commit --amend
  3. to push the updated commit to the repo, do:

    git push -f

Really, consider using the preferred method mentioned before.