Where did you read it is the switch? More likely it is the tank. Is there a pressure gage by the tank or anywhere else? Watch it for a few cycles. If the pump kicks on at about 30psi and quickly climbs to 50 and cuts off, rapidly drops to 30, and kicks on again about 30, the switch is doing exactly what it should. The tank has lost its air. Most of them have a bladder or diaphragm to retain the air, but they can be damaged. See if the tank gives instructions for adding air. If not, shut the power off to the pump at the breaker box if no other switch. Open a faucet, the lower the better, perhaps outside and let the water run out. Close the valve at the tank. Locate the air valve, and add air to bring the pressure up to 2 psi below the pressure the pump kicks on at. Turn the pump on and let it pump until it kicks off. Open the valve from the tank. See if once the air is out of the lines, the system works like it used to. If so, you can either plan to add air occasionally or buy a new tank that has a working bladder.
If the gage instead changes very little from where the pump starts and stops, the pressure switch isn't working right. Unless somebody fiddled with it when things quit working right, likely it is bad. They aren't that much and any hardware or home center should have them. Shut the power off, shut the valve to the tank off, and replace it.
If you don't have a gage, or it doesn't work, again any hardware or home center should have them. If there isn't a gage, you will need a 1/4'' tee and short nipple to add one next to the switch.
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