Gently prise off tone knob with small flat end screw driver. Working your way round till it pops off.

Remove 10mm lock nut with socket. Should only be finger tight so should come off easily. With the amp lying on its back squirt a very small amount of penetrating oil into the shaft / switch. Gently and carefully push the shaft / switch to the side so that oil can work it’s way around. Leave for a minute or two.

Now with a flat ended screw driver (about 7”long for leverage) carefully but firmly try to turn the switch. Should come free at this point.

Mine was stuck for over 10years and it freed up easily. Wipe away any excess oil and reassemble.