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 (-p)

git log -pS 'some string to search' your/file/or/directory

Clean up untracked files

git clean -fd