Sage Green Tiered Maxi Dress

I bought this dress for a wedding but it's too big and I can't return it or exchange it for another size because it's final sale. Here's the description from the website, "Lightweight woven fabric, with a texture floral design throughout, shapes this dress that has adjustable spaghetti straps, a ruffled straight neckline with elastic, and a lightly gathered bodice. Tiers of elastic create a smocked effect at the waist above a tiered maxi skirt."