Back out the screws or remove the bolts under the steps, which hold the existing treads to the tread supports. Remove the old treads from the stairs.
Set the wooden replacement steps onto the tread support. Center the steps, measuring from the wall to make sure they all line up. Position the back edge of the steps against the axial stair support beam.
Mark the screw holes through the tread supports on the bottom of the new steps.
Remove the steps to drill pilot holes in the bottoms. Use a drill bit with a slightly smaller diameter than the screws.
Replace the steps onto the tread supports, lining up the holes in the tread supports with the pilot holes in the steps. Drive screws through the tread supports and into the steps to secure the steps in place.