GIT: Undo changes from a specific file

Undo the changes and set it back to master:

git checkout origin/master -- /path/to/file

Another posibilities:

git checkout v1.2.3 -- /path/to/file         # tag v1.2.3
git checkout stable -- /path/to/file         # stable branch
git checkout origin/master -- /path/to/file  # upstream master
git checkout HEAD -- /path/to/file           # the version from the most recent commit
git checkout HEAD^ -- /path/to/file          # the version before the most recent commit