How do you reset a mymeeba?

You can't.

Actually- you can.

You press the tiny reset button on the side of the my meeba sliding part with a sharp object. Slide the sliding thing down for 2 seconds, then slide it up for 2 seconds. Now a new baby my meeba will appear.

What you can't do is lock the lid. It will remain unlocked once the original game is completed (unlocked).