Merging tags from Admin UI?

I’m working on a site where a previous user created a lot of unnecessary/duplicate tags (the same phrase spelled differently, etc). Is there a way to merge two tags within the admin UI? If not, does anyone have hints on how to do this via a DB query on the back-end?

FWIW I tried simply renaming one of them to the same name as the other but the rename didn’t “stick”