How do I repair a dripping Moen M4600 (1978) bathroom faucet? Looks like the Moen 1 handle 64621P that is pictured here. Thanks. Richard

First shut off water supply and remove cover on top of handle.They generally just snap in place so you can remove with screwdriver or knife point. Remove screw in center of handle and pull handle off. There is a copper retaining clip that holds cartridge in place. It has a small hole in the center on top that you can insert scratch awl or other pointed tool in and pull straight up to remove.You can try to grasp center stem of cartridge with pliers and pull up to remove. If the cartridge has not been changed for a long time it may be very hard to remove.
You are going to have to replace the entire cartridge so you should have one on hand and you can purchase a tool made especially to remove moen cartridges.Just follow direction that come with cartridge or tool and remove cartridge. I would use the brass cartridge as I have not had good results with the plastic ones. When you get old cartridge out I would use a small amount of faucet grease or petroleum jelly to lube o-rings and then reinstall new cartridge and clip,handle,screw,and cover. Good luck and thank you.