A user edits a tag wiki for a tag with 1 question on a whole SX site, and this comes up for my review. Now, the edit in itself may be fine, but what should I do if what I think should happen is for the tag to be deleted? There's no button for it.
In order for a tag to be deleted you simply have to remove it from all the questions it is associated with. This process is known as Burnination. 24 hours after that the system will delete the tag automatically provided no questions are tagged with it in the meantime.
Normally one would post on the site's meta first to check whether the site's community is supportive of such an action. Tag the meta question with burninate-request so that the community is aware.
Retagging is a manual process because each question must be assessed to see whether replacement of the offending tag with some other tag or tags rather than removal is the best course of action. Also if you retag something you're supposed to check that it doesn't have any other issues e.g. spelling/grammar.