Identify the shifted stair wedges on the underside of the squeaky stairs. Look for wedges that have a space between the tread and riser. These are the problem wedges that are causing the stairs to be noisy.
Slide a flat bar under the wedge and carefully pry it off from the stair stringer by pulling the bar outward.
Pull the finish nails out of the wedges with diagonal pliers. Fit the wedges tightly against the stringer so the top edge is tight against the tread and the back side is tightly against the riser.
Drive one wood screw through each corner of the wedges into the stringer using a screw gun to secure them in place.