You'll want to "load test" the starter switch. With the switch held in the start position, both terminals of the switch should have voltage.
If neither side has voltage, there is a problem ahead of the switch, like a loose battery cable, low battery, bad battery, etc.
If the battery side has voltage and the starter side doesn't, the switch is bad.
If both sides have voltage, and the starter isn't cranking, there is a problem in the starter, or the cable between the switch and starter is loose or bad.
To change the switch, remove the 4 bolts, lift the switch. Tie a wire to the linkage so it won't fall down, remove the linkage pin.