Dead easy to chnage rust its the only issue.
Undo the two bolts holding the ball joint to the arm, its a 17mm spanner on top and 16mm nut on the bottom from memory.
Then undo the bolt which holds the ball joint into the hub it can be a pain to get out but douce it in penetrating oil however rusty it is.
Remove the bolt, pull the ball joint out replace with a new one then you'll have to fiddle aroung moving bits so you can tigthen the two lower bolts up.
Best use the new bolts which should be supplied with the ball joint and even better use locking compound as well and tigthen them to the correct torque.
Shouldn't take more than half and hour ro do took me about 3hours on a Williams once due to rust though as I have to take the whole hub off.