Turn off the circuit breaker and verify the power is off with a voltage tester
Remove the switch cover plate and unscrew the existing switch
Identify the three wires:
Common wire
Traveler wire 1
Traveler wire 2
If the switch is a 3-way switch, connect the common wire to the dark-colored screw
Connect the two traveler wires to the two brass-colored screws
Tighten all terminal screws securely
Attach the ground wire to the green ground screw if present
Mount the switch back into the electrical box
Reinstall the cover plate
Turn the breaker back on and test the switch