I need to change my apple watch band size

If you recently bought an Apple Watch from Apple or an authorized reseller, you can return it for a full refund and get one with the band size you need.

If it's been more than 14 days, then nothing can be done except buying a new different sized band entirely.

Do note that if you bought an Apple Watch, which comes with a band and the size of the band is not right and you are still within the return window, the ENTIRE package has to be returned, which includes the watch, all accessories and they have to be in the original packaging you received them in.