It's easy to do. Unscrew the bottom two screws, remove the bottom plate, take a good look where the wires are exactly. Pull the wires off the motor, pull the motor out. Unscrew the screws inside the grip, pull the grip off carefully. Put the new grip on, arrange the wires and put the motor in. Connect the wires, screw back the bottom plate.
Now adjust the motor height. Without magazine or BBs, fire the gun. Be careful, there might be one BB left in the barrel.
If you hear loud screeching and the gun doesn't fire, turn the motor height screw in, the motor doesn't reach the gears.
Turn the motor height adjuster screw in, until the gears turn a bit slower or stop. That's too high. Lower the motor bit by bit, until you find the most quiet height. It is just below the "too high". If you lower it even further, the gun still fires, but the gears are louder, because they are connecting too loosely.