The motor just twist off the body of the filter. then you just pool off the old shaft and replace with new none. just make shore when you pout the motor bake the rubber o ring is still in place.
drain the water out first, twist off the motor, gently pull out the old impeller, clean out the impeller housing with a qtip, replace with new impeller, make sure the o-ring is still ok (might be a good idea to order a new one "just cuz"), reinstall the motor, fill with water, check for leaks, plug it back in