A reducer is used to remove silver from a negative or print. For an over exposed negative you need a cutting reducer like Farmer's Reducer. After developing and fixing the negative wash it briefly and then immerse it in Farmer's reducer. The formula is readily available on many websites and consists of potassium ferricyanide and sodium thiosulfate. Watch the negative carefully to check when the density is approaching the desired level. Remove the negative from the bath. Rewash the negative for your usual time to remove any thiosulfate from the reducing bath. It is best to stop just before the negative reaches the desired density as reduction continues for a brief period during washing. This can all be done in room light.
Kodak used to supply this reducer in small foil pouches for single use. An older photo store might still have it. You can buy the reducer ready to be mixed from Photographer's Formulary. You want their Reducer #1. This makes more stock solution than you need but it is handy to have and works on prints as well.