First, carefully let out the hem using a seam ripper or a pair of scissors. Make sure the bottom edge of your skirt fabric doesn't fray by running a zigzag stitch across the bottom (if the manufacturers haven't done this already).
Next, stitch a length of ribbon to the bottom of the skirt. The ribbon should closely match the colour and weight of the dress. I used a fine polyester ribbon with a width of 1.7 cm. Pin the ribbon to the dress so it just covers the zigzag stitch and sew it down.
video on blind stitching.