Shortening the Steps
Locate the bottom end of the stringer. Measure up from the bottom edge of the stringer the distance that you would like to shorten each stair. This is the portion that will rest on the floor at the bottom of the staircase. Make a line parallel to the stringer bottom, spaced from the bottom edge the distance you want the step shortened.
Measure down from each tread, or horizontal support on the stringer and mark a line parallel the same distance down as you measured up from the bottom of the stringer. Make a perpendicular line in line with the riser face to create the new back corner of the stringer support, extending the riser down to meet the new tread height. The treads are the angles that are parallel to the bottom of the stringer.
Cut the bottom of the stringer off along the line you drew with a jigsaw. Cut each tread along the line, starting with the second tread, since the first has already been shortened when the bottom of the stringer was cut. Cut down the line extending the riser to remove the cut piece.
Raising the Step Height
Cut ¾ inch hardwood strips, as wide as your stringers are thick -- typical stringers made of 2-by lumber are 1 ½ inches thick -- and as long as the tread supports are deep. Cut one for each tread and one for the bottom of the stringer. Use a table saw to cut the pieces to width and a miter saw to cut them to length.
Apply wood glue to the bottom edge of the stringer. Fit one of your hardwood strips to the bottom edge, flush with the front edge and both faces of the stringer. Nail through the strip into the bottom of the stringer. Use a pin nail gun and 2 inch nails. Use 4 nails in each strip.
Glue and nail a strip to the top of each stair tread support, raising the height ¾ of an inch, so that each step will be ¾ inches taller than originally designed. Use thicker or thinner material as required to make the necessary adjustment.