H&M question :) for returning something?

I bought a shirt frm H&M but its the wrong size. I check the website to see the name of the shirt and check to see if it matched the name on the receipt but it didn't so I'm not sure if I have the receipt as I'm in the process of finding it. Can I be able to return it without a receipt? I bought it like a week or a week & a half ago. What would happen of I return it w/o receipt? I don't want to exchange cause it's not in stores anymore & sold out online ): so I jst want my money back or a store credit I don't mind.Thanks for the feedback! Would appreciate ur help as I nvr returned something w/o a receipt before.