The well water pressure increased by itself to 80 - 90 psi. What would cause this? Could I just replace the switch?

You can try adjusting it down by turning the large center nut counterclockwise. Would need several turns. However, I would also check the little pipe leading to the switch. These get clogged with "crud" rather easily and make the switch respond slowly.

I'd watch that pressure carefully. 90 is getting fairly high. Switches are cheap and relatively easy to replace.

If the control changed by itself then something's happened internally, A spring that weakened, etc.
I would check the small tube first and if it's clear then I'd replace the control box. Good luck, Tom