Turn off power to the circuit at the breaker
Verify power is off using a non-contact voltage tester
Remove the switch cover plate
Unscrew and pull the old switch from the wall
Photograph the existing wiring or note which wires connect to which terminals
Loosen terminal screws and disconnect the wires from the old switch
If there are ground wires, disconnect the ground from the old switch and note where it connects
Check the new switch is compatible (single-pole vs 3-way, voltage rating, amperage rating)
Connect wires to the new switch using the same configuration as the old switch
Tighten terminal screws securely
If the switch has a ground terminal, connect the ground wire to it
Carefully push wires back into the electrical box
Position the switch so it sits flush with the wall
Screw the switch into the electrical box
Reinstall the cover plate
Turn the breaker back on
Test the switch operation (on/off and, if applicable, dimming or 3-way behavior)