You need to know your commit hash from the commit you want to revert to. You can get it from a GitHub URL like: https://github.com/your-organization/your-project/commits/master
Let's say the hash from the commit (where you want to go back to) is "99fb454" (long version "99fb45413eb9ca4b3063e07b40402b136a8cf264"), then all you have to do is:
git reset --hard 99fb45413eb9ca4b3063e07b40402b136a8cf264
git push --force
