You can return diapers anywhere even without a receipt.
Well actually almost anywhere as different stores will have different policies.
However as long as the store that you return the diapers to sells that brand of diapers then most often they will accept the return for a store credit or for an exchange of a different diaper size.
Some people have returned diapers to a store to exchange them for a different size when their child needs a larger size diaper.
As long as the diapers are not opened they can be exchanged and returned for store credit or for a different size diaper.
If opened however the store will not take them back.
The diapers must be sealed and not opened before they will take them back.
If you have a receipt then you may get your money back but if not then you will get store credit or can simply exchange the diapers for another brand or another size.
However if the other brand is more expensive then you'll have to pay the difference.