Turn off the power to the air handler at the breaker box.
Pull the top of the thermostat off the wall.
Remove and label each wire attached to the bottom plate. Label wires according the code on the terminal from which it was removed.
Remove the bottom plate completely. Mount the new bottom plate.
Connect the wires according to the labels on the bottom plate. For example: Y for yellow wire, W for white wire, R for red wire and G for green wire.
Snap on the thermostat to the bottom plate.