How do I fix elastic that is around the top of my dress, one side is stretched and the other is not.
I assume that you are refering to the neck of your dress & that the elastic is all in one piece. I would try to open the neck seams that are enclosing the elastic, pull out the stretched part. Making sure to fasten the good piece with safety pins on both ends at the seams so you do not loose the elastic back into the casing. Cut off the stretched part. With a safety pin attached to the end of a new piece of elastic thread the pin that is attached to the elastic into the channel that the old elastic was in. When you get it all the way through to the other seam, pin it so that it doesn't slide back. ( I have 'lost' the elastic many times till it Dawned on me to pin the ends so that it wouldn't happen.) Then adjust the cloth along the channel until it's the size you want. Pin it in place before you cut it!! Sew the ends of the elastic (all pieces) to the side seams that you had opened. Sew up side seams. But if it was me, I would remove all the elastic so that it was all new, so that I wouldn't have to repeat the process again.
If it is part of the dress, meaning it is sewn right into the fabric, and not in the seam as mentioned in the above tip, then you can get a new piece of elastic, and sew it right over it. As you sew it on, be sure to stretch the elastic as you go, so that it has the ruffled look. But not sure what/how the elastic is in your dress. Good luck.
Laura in NH