Not sure how to change the switch but some knobs are retained by a small grub-screw or a spring clip and it is sometimes worthwhile checking the switch shaft is actually being turned by the knob.
call the kenwood company spares department first and ask if spares are still available for that model, then ask for the address of a service agent in your area
if there are no spares then sending it any where for a service may get a reply that it is unrepairable because of lack of spares
extremely unlikely as manufacturers stop carrying parts after 5 years and aftermarket suppliers and service agent after 10 years
you may get gears from alibaba site but the time taken , makes junking it for a new unit a better idea
the head gears are not engaging properly
could be the selector knob or stripped gears
get a quote from an appliance repair shop but if the mixer has a few years on ti then it will probably turn out cheaper to get a new one