Let me simplify it for the users that don't understand git very well, like me :D
If you want to unmerge the commit and remove it from the remote git as well. Follow:
git reset --hard HEAD^ or
git reset --merge hash-of-commit-before-merge i.e. c4ab5bcc4ab5de Now you have successfully reset on your local, it's time to update/sync this on remote
git push --force // this will remove the merge or afterwards commits 