Unscrew the bottom of the cooker, identify the beeper (looks like a small black cup, pull the sound membrane out of the beeper with tweezers, screw it shut.

No soldering iron or knowledge of electronics needed.

But beware: this may invalidate the warranty.

Sorry. Do not unscrew the bottom of the cooker but the base.
Be sure to disconnect the mains plug!

It's been a while since I operated on mine.