How to delete the last n commits on Github and locally?

To remove the last two commits locally I'd suggest using:

git reset --hard HEAD^^

Rebase is a completely different operation that won't help you here.

If you want to remove the 2 (two) last commits, there is an easy command to do that:

git reset --hard HEAD~2

You can change the 2 for any number of last commits you want to remove.

And to push this change to remote, you need to do a git push with the force (-f) parameter:

git push -f

However, I don't recommend to do any git command with -f or --hard options involved if there are new commits on remote (Github) after this commits that you want to remove. In that case, always use git revert.