Remove sprocket. take a couple of wood screws and screw them into the center of the seal (between the outer and inner edge and not too deep). Prise the seal out with pliers or the claw of a carpenter's hammer. I'd put a thin film of oil on outside of the new seal (just for extra assurance it seals on the outer edge and stays put) and a film of oil on the inner lip of the new seal. Gently tap the seal squarely into the case. Replace sprocket, install chain.