You don't prime a well, you prime a jet pump and if there is a leak from the pump back down the well to the foot valve, you will lose prime.
To replace a foot valve you find the well, open it by removing the seal/cap and disconnect the plumbing and lift the drop pipe that goes down the well, out of the well. The foot valve is on the end of the drop pipe. If you have a deep well two line jet pump there will be two lines down the well and a j-body on their ends and then the foot valve on the j-body.
The foot valve can be from as little as 20' to 150' deep depending on the well and pump. The drop pipe can be plastic or galvanized pipe and too heavy to remove by hand. I suggest you call a pump guy, well driller or only a plumber that knows wells and pumps (most don't).