After some use, the rubber feet can gather dirt and dust (depending on how dirty your desk is).
To clean them, you need to remove the feet completely.
Clean the adhesive on the feet and the keyboard's base with alcohol.
Use 3M double-sided tape or some kind of strong glue, preferably contact glue or AB glue to stick the feet to the base. Do not use Loctite or superglue because this can damage the base and the mylar material of the keyboard.
If you use double-sided tape, make sure the tape follows the shape of the rubber feet. Be careful when doing this. Also, be careful not to stain the bottom mylar sticker or damage the rubber feet.