Undo a mysql UPDATE command

If you committed your transaction, it's time to dust off that backup, sorry. But that's what backups are for. I did something like that once myself... once.

Just an idea - could you restore your backup to a NEW database and then do a cross database query to update that column based on the data it used to be?

Nothing.

Despite this you can be glad that you've got that learning experience under your belt and be proud of how you'll now change your habits to greatly reduce the chance of it happening again. You'll be the 'master' now that can teach the young pups and quote from actual battle-tested experience.