Liz, its pretty simple.
Do you have a sewing machine & a overlocker?
You need to unpick the stitching then unpick your dead elastic, making sure to mark where it was was. Place your new elastic on with the sewing machine then close it all back up with the overlocker.

I would be more then happy to do this for you if you want to send them my way.
eta - I don't want to paid to do it. Just cover your postage costs.