How do I safely remove my old thermostat before installing the RTH5160?

First, turn off power at the breaker box or switch controlling your heating/cooling system. Then test the system by setting the temperature above room temperature in heat mode or below it in cool mode—if it doesn't turn on within 5 minutes, power is off. Remove the faceplate carefully without disconnecting any wires yet, and take a photo of the wiring with terminal letters visible for reference.