If the adhesive separated from the foot:

Slide the flat end of a spudger under an edge of the adhesive. If the adhesive seems stuck, you may need to heat the adhesive further.

Use a pair of tweezers or a spudger to remove any large chunks of leftover adhesive.

For installing new mouse feet or reusing your old ones:

Thoroughly clean the bottom of the mouse with isopropyl alcohol and a lint-free of microfiber cloth. Allow the surface to dry completely.

Peel your new mouse feet away from their adhesive liners and press them firmly into place.