Insert spudger between button and outer white ring.

Twist the spudger to separate the ring from the body of the mouse. Work the spudger around the whole mouse until the ring is completely detached.

You must use some force to break the adhesive that holds the ring in place, but try not to crack the plastic ring.

You will have to glue this ring back to the mouse body when reassembling it. Cyanoacrylate adhesive works well.