You may have to replace stair treads because they are damaged, or you may want to replace stair treads because of the availability of a better type of wood. Either way this task is a straightforward operation that involves the removal of the old step and replacing it with a new one.
Remove the old stair tread. In most cases this is as simple as taking a flat metal pry bar and driving it between the riser and the tread and slowly raising the tread straight up. Do this a little bit at a time and be careful not to make any marks on the surrounding risers. After a while you will be able to get your pry bar between the stringer and the tread. When you do, keep prying until the tread comes free.
Another possible obstacle to removing the tread is the skirt or wall board that is located along the wall. Sometimes this piece of trim has to be loosened or removed first, but often the tread will pull free without any adjustments to the wall board.
Remove any nails from the stairs or the tread.
Measure and cut a new piece that is the identical size of the piece that you just removed. Also, you will want to match the wood if at all possible. Be aware that many stair treads are made from oak or maple, so you need to get an exact match or as close as possible. If you use a different type of wood, the step will wear at a different rate from the other stairs, and this could conceivably cause a safety concern. Don't forget to sand and plane the outside edge so that it is rounded like the old tread.
Pre-drill holes into the tread wherever there is going to be a nail to hold the tread in place. The pre-drilling process is especially important if you are working with a hardwood. Make sure the drill bit is slightly smaller than the shank of the nail.
Carefully nail the new tread into place and set each nail. Then fill with a colour-matching wood filler or putty.
Add your finish and let the tread try thoroughly before use. Sometimes wooden steps may need several coats of finish.
If replacing more than one tread, treat each tread as an independent project. If there is a banister present, the banister will have to be either loosened or removed. This can be a very involved process.
Make sure the nailing surface below the tread is solid and in good shape. Be certain the stairs are not used (or a temporary tread is installed) during the process of making the new tread.