We recommend both throwing wheels be replaced even if only one is substandard. The machine should be powered off when performing the replacement. To replace the top wheel pull the wheel towards you with your hand as you push the bottom wheel away from you (and vice versa when replacing the bottom wheel). This provides you with easy access to slide the old wheel off. Then use a 3mm allen key to loosen the set screw on the right side of each wheel counter-clockwise until the wheel is loose and then slide it off. Note there is no need to completely remove the set screw. Then slide the new wheel on. Esure that there is about 1mm of space on the left side of the rubber wheel so it does not rub against the motor. If metal motor axle on the right side is flush with the face of the rubber motor that should leave you the appropriate space on the left side. Ensure that the set screw is tightened over the flat part of the metal axle ‘D’ profile so that it secures the wheel properly. Otherwise if tightened over the round side the wheel will skip around the axle. Once both wheels are replaced you can use your machine again.