Warning: This can be really bad to do. It changes your git history, which is bad and could affect other remotes. Don't do this to a project that other people are using too. Now that you've been warned, here's how to do some bad things to your repo. Do it quickly before anyone notices.
Roll back 1 commit locally:
git reset --hard HEAD~1
Roll back 6 commits locally:
git reset --hard HEAD~6