Prior to replacing the starter or solenoid ensure the battery connections are clean and secure and that the battery itself is "good".
To replace the starter contacts, the starter must be removed (with the bettery disconnected). At the top of the starter is a cover secured by three small bolts (8mm wrench) remove the cover and you will see a plunger and spring that slide out. The contacts are two "L- shaped" copper items. You need a 14mm wrench to remove them, when replacing them ensure the insulation is replaced in reverse order as you had disassembled it and bench test the starter prior to putting it back in the car.