Turn off the power to the air handler at the breaker.
Remove the top of current thermostat. Remove and label each wire on the old subbase (the bottom part screwed to wall) of the thermostat with the letter code found on the terminal from which it exits.
Withdraw the subbase from the wall.
Pull the wires on the wall through the opening in the new subbase and then mount the new subbase on the wall.
Insert the tips of the labeled wires to the terminal that matches the label on the wire. Use a small piece of wire to attach one end to the "X2" wire and the other end to the "W" wire and then insert each wire into its own terminal. Tape the "Y" wire with electrical tape and do not connect it to anything.
Attach the thermostat to the subbase.
Turn the power back on.