Remove the end-nosing and balusters on the damaged stair treads.
Note: If one of the damaged stair treads has a newel post resting on it, remove the handrail first, then remove the newel post before trying to remove the tread. This will allow the tread to be removed without additional damage.
Remove the damaged stair treads by prying them loose from the riser and the staircase stringer beams.
Caution: Any removed stair treads represent a danger to anyone wanting to use the stairs. Cordon off the staircase until the treads are replaced.
To salvage parts of stair treads, assess the damage to it. If feasible, cut off the damaged portion of the stair tread, and cut a matching portion from another damaged stair tread to fill the portion cut from the first damaged tread.
Use a plate joiner on the tread edges to be assembled, place corresponding reference points on both pieces of the tread and cut the slots with the plate joiner. Insert the biscuits into the slotted cuts with glue, then glue and clamp the two tread sections together using bar clamps.
Note: Oppose the bar clamps to avoid cupping on the newly repaired tread.
If no other damaged tread is available, use new tread material. New stair treads are available in the typical one inch stair tread thickness and with the edge already rounded over. Cut the new stair tread to length, matching the original damaged tread.
If the damaged tread has a curved end, use a saber saw to cut the radius (Image 1) on the new tread material. Also, be sure to create a matching bull nosed edge on the radius cut. You may need a special router bit to create the bull nose edge (Image 2).