Change user ID advice please

[The Helmet]

Hello
My users are winjing at me. I can't seem to find it anywhere and I suspect I am just being dim. They need to change there user ID names but unless I manually create another user and merge it which seems laborious I can't seem to do it. Perhaps I can't see the wood for the trees?. In PHPBB you could just change your name and that was it. Have I missed something?