How can I return a wrongly measured item and get a refund?

[Premchand]

I ordered an item with https://fashiondealindia.myshopify.com/products/pack-of-3-comfortable-convenient-front-button-bra thru an Instagram ad.  The delivered item was not as per the item ordered. How to reach that seller. There is no confirmation mail, no contact number and no receipts. What is to be done to return the item and get my money back

Please Help!!!