Overhaul as in pull the bearings out, clean them with degreaser, then inspect them. If the bearings are not pitted and the retainer isn't bent out of shape, you can reuse them. If they are messed up, they are very cheap to replace, many bike shops will have replacements on hand. Once you have cleaned or gotten new bearings, pack the cups they sit in full of grease, and reassemble. Typically bearing retainers will have a side that is more rounded and a side that is more flat, you usually want to put them into the cup rounded side facing in.
Once you have it back together, you want to adjust it. The goal is to have the bearings be as loose as possible while not having any play in them. You also want to make sure the locknut is tight against the cone.