This isn’t actually GitHub specific but I wish they’d add a button to make it easy!
Anyways, here’s what I use to undo changes to a given file:
git diff origin/master... your/file/here.txt | git apply --reverse
Some other useful git commands
Rebasing master onto current branch without switching
git fetch origin master; git rebase origin/master
Searching git history for a substring
git log -S 'some string to search'
Which you can also give a path and include the actual diff (
git log -pS 'some string to search' your/file/or/directory
Clean up untracked files
git clean -fd