The feet are secured to the bottom of the mouse with strong adhesive. In order to cleanly remove them, the adhesive needs to be thoroughly heated.

Heat an iOpener and lay it on the mouse feet for one minute to heat the adhesive.

Alternatively, you can use a hair dryer on the Low heat setting to soften the adhesive.

If you're using a hair dryer, only heat the feet a few seconds at a time or you may warp the plastic shell.