Or how to reset a merge commit?

If just after a merge commit you recognized that it was actually “git rebase” that you wanted to do, then your friend is:

git reset –merge 14c1d90c3e

where the target commit is the one preceding the merge.

But when reset is not an option?

But if you found the faulty merge after several other commits (or after pushing to remotes), resetting is not so good idea or won’t work at all. In that case refer to How to Revert Faulty Merge in the HowTo.