Ugh, it’s ridiculous. I’ve done this plenty of times but when trying to answer this question, I couldn’t remember how, and looking around the interface, it took me probably a whole minute to find it again. It’s waaaay up in the header — you know, with all the things not related to the post.
It’s also not obvious how to edit tags. There’s no edit option beside them, and they go away in edit mode. I gave up on figuring this out, but when I hit Publish it popped up with an interface where you can edit them. So at least that’s discoverable… just not when looking for it.