Mark the position of the old stair rail on the lower deck post using a pencil. The marks serve as a reference point when installing the new rail.
Remove the old stair rail screws using a power screwdriver. Turn the screws counterclockwise. Have a helper hold the rail as the screws are removed to prevent it from falling.
Position the new stair rail on the deck posts aligning the rail with the upper rear corner of the top post and the mark on the lower post. Ask the helper to hold the rail firmly in place.
Attach the stair rail to the posts at the top and bottom with two 3-inch galvanized deck screws. Tighten one screw at each end clockwise for temporary alignment.
Mark a line on the back side of the stairway rail where it meets the post using the pencil. This mark is the reference line you will use when cutting the stair rail to the proper size.
Mark a line at the bottom end of the stair rail using a level. Place the level vertically against the lower inside surface of the rail at the desired distance from the post. Mark a vertical plumb line with the pencil.
Remove the two screws from the stair rail. Have the helper hold the rail as the screws are removed.
Set the stair rail across two sawhorses. Cut along the marked lines on the rail using a circular saw.
Align the stair rail with the posts and secure it with deck screws. Have the helper hold the rail in place until the upper and lower screws are installed.