1) Prior to disconnecting the wires, 1st turn the circuit breaker to OFF
2) Located on the existing 3-way, you will have (1) one black screw, (2) two brass screws and (1) one green screw. The black screw is the common terminal and the 2 brass screws are for the traveler wires. The green screw is for the ground wire.
3) Remove (1) one wire at a time from the old switch and replace with the appropriate color wire onto the new programmable switch
4) The wire marked as "C" is the common wire and this gets spliced to the wire that was on the black screw of the existing switch.
It is important that you wire the correct house wire to the new switch "C" wire.
5) The other two wires marked as "1 and 2" are for the traveler wires. One of these wires will get spliced to one of the wires that you removed from the brass screw. The 2nd wire will get spliced to the other wire that you removed from the brass screw.
6) I am looking at the installation directions and they do not mention a ground wire on the programmable switch. The diagrams do not show a ground wire so I assume there is no ground screw or ground wire on the new switch.
If your wall box is a metal box, terminate the bare copper ground wire into the threaded hole located in the back of the metal box and use a green grounding screw.
If your wall box is a PVC, you may have other bare copper ground wires. If so, splice all the bare copper ground wires together.