Changing two prong electrical outlets to grounded three prongs
It sounds like you have an equipment grounding conductor (EGC a.k.a. ground wire).
If so, you can use either a self grounding receptacle where the connection to the box is a legal ground (receps must be listed as self grounding), or you could attach a ground wire to the green screw on the recep and attach it to the box.
If you are comfortable opening your electrical panel, you should do that and check to see if there are actually 3 conductors coming from those receps. That is to verify what you area seeing is actually a grounding wire.