How do I change both a cartridge and stylus for a Realistic Lab 390 Direct Drive Turntable?

The complete head assembly is held in place by the metal collar at the back of the head. Turn it counterclockwise looking from the back (along the arm). It should only turn about a quarter turn and then the head unplug. The cartridge itself is held in place by two screws. The electrical connections by four pigtails which should just pull off the back of the cartridge, but be careful, they are thin. Pull them off while the cartridge is still fitted but it is easier to plug onto the new one before bolting it down. Wiring is top pins to top pins, bottom to bottom but left to right cross over.