git reset --hard HEAD~1
git reset --merge HEAD~1
git revert -m 1 <merge-commit>
git revert -m 1 commit_hash
git reset --hard HEAD~1
git reset --hard <commit_sha>
git reset --hard <cbm>
where cbm is the commit number before the merge you want to revert
to get a list of all the merges you can use the following line
git log --graph
and then use the commit number from the one before the merge you want to revert
// find the commit hash
git log --oneline
git revert -m 1 [commit-hash]
// https://www.datree.io/resources/git-undo-merge
git reset --merge origin/master