Magit allows renaming various Git references. I can't see how to rename a tag with Magit commands.
Tags cannot be renamed. Git itself does not offer a command for doing it. Magit could of course implement this by calling git multiple times, but I think it actually makes sense that tags cannot be renamed. Also this would only work for non-annotated tags anyway. I don't remember the details, but renaming an annotated tag confuses certain git commands. Git handles tags quite differently from all other refs.
So instead just delete the old tag and then create a new one.