Find the ref spec of the commit you want to be the head of your branch on Github and use the following command:
git push origin +[ref]:[branchName]
In your case, if you just want to go back one commit, find the beginning of the ref for that commit, say for example it is 7f6d03, and the name of the branch you want to change, say for example it is master, and do the following:
git push origin +7f6d03:master
The plus character is interpreted as --force, which will be necessary since you are rewriting history.
Note that any time you --force a commit you could potentially rewrite other peoples' history who merge your branch. However, if you catch the problem quickly (before anyone else merges your branch), you won't have any issues.
