If you already pushed your changes, you will need to force your next push after resetting the HEAD.
git reset --hard HEAD^
git merge COMMIT_SHA1
git push --force
Warning: a hard reset will undo any uncommitted modifications in your working copy, while a force push will completely overwrite the state of the remote branch with the current state of the local branch.
Just in case, on Windows (using the Windows command line, not Bash) it's actually four ^^^^
instead of one, so it's
git reset --hard HEAD^^^^