Turn the skirt inside out and use a seam ripper to take out the existing hem. If you're taking the skirt up more than 2 or 3 inches, just cut off the hemmed part of the skirt completely.
Place a ruler straight across the area of the skirt where you want the new hem to be, and mark the area with chalk. Don't worry about marking underneath the pleats; we'll get to that in a second.
Iron the pleats flat as best you can and extend the chalk line to match in between where the pleats were. You're on your way to a new sexy wardrobe item!
Cut the excess fabric off the bottom of the skirt, leaving about an inch of fabric below the chalk line to account for the hem. You don't want to make the skirt too short!
Fold the fabric along the chalk line and iron the fold in place. This will create your new hem.
Sew the new hem in place along the raw edge of the fabric, tucking under the raw edge as you sew. Press the new hem, then fold and press each pleat into place. Turn the skirt right side out. Now pair it with some knee socks and hit the town!