take it apart- wet it= press it carefully= being very careful to not stretch it again...and resew---very carefully-with lots of pins- slowly...if it is stretched too out of whack you will need to cut new pieces- bias edges really take alot of care- not to (iron) but to only press- lots of pins- slow stitching making sure to not stretch as you stitch.
it is alot easier to when possible use a square, stitch the diagonal then trim- so you don't have to handle those bias edges- but it is not always possible