remove the stitches very carefully, remove the elastic. then shorten it or buy elastic to lengthen it. sew the skirt leaving room for the elastic, then pull the elastic through the casing with a safety pin on one end of the elastic to 'thread it' through. then sew the small opening left to thread the elastic. it would help to get a sewing book from the library to see diagrams of the process. if the skirt is too long, then you can measure the necessary amount from the top of the skirt with a ruler, mark it with chalk, cut off the top including the elastic and make a new top for the elasic to fit in. this is the easiest way.