Well ... you would need to remove the case back, unscrew the crown, pull the crown outwards to the Time Set position, press the stem release button with a properly-sized screw driver, remove the crown and stem then MOST LIKELY the a21j will fall out in your hand. Then you need to place the case and crystal in a container to keep the dust out.
Secure the movement, dial up, in a movement holder. Place a dial protector between the hands and dial. Use the hand pullers to remove the hands. Detach the dial from the movement .. little hooks on the movement grab the dial's legs.
Reverse this to use a new movement. Use a Hands Presser to re-attach the hands.