replacing receptacle

I have old 2 prong outlets in a house I just bought. Iwant to change them to 3 prong, when I took the covers off and looked at the outlets there are just two wires connected to it.

the new outlets have 2 screws on each side and a green screw, would I just hook up the wire the same way it was on the old outlet.

also i looked in the back of the outlet box and there is a screw with a wire attached to it, would that mean the box is grounded. also what do I do with the green screw since I have no wire to attach to it.

thanks for any help you can give me
Donna

 

If the boxes are indeed grounded (and you need to verify the ground) then you need to run a ground wire to both the box itself and the green screw on receptacle. Use wire nuts and pigtails for this.