Looks like your seal has a nick in it. If you need to replace it you will need a pair of snap ring pliers. I would recommend buying a pair that does both inside and outside with the right size of tips. Spray the area with WD40 or the like and soak it. If the snap ring comes right out then great. If not, then use a hammer and punch to gently
try to free the snap ring. Set the new seal in using a socket or the like that fits right to the edge of seal. Put grease thick into the spring area on the inside of the seal to keep the spring from popping off.