Apple watch band

If you bought just the band (meaning not a watch and band at the same time), take a copy of the receipt with the band packaged in the original packaging and take it back to where you purchased it. You have 14 days from date of original purchase to return it for refund, then buy the band you need.

Do you have an Apple Store near you? If yes, go there and return the band.

If you ordered the band from Apple and it was delivered to you and visiting an Apple Store isn't an option for you, see this link about returns --> Returns & Refunds - Shopping Help - Education - Apple