Change Apple Watch band size

we can't, we are not apple

This is a user forum, you are not addressing apple here  just  fellow users

 Returns & Refunds - Shopping Help - Apple

you can try contacting apple Support ->  Get Support (apple.com)

and or give them feedback -> Product Feedback - Apple

in some countries there is always a 14 days return full money back option

if it was me then I would try contacting them

https://getsupport.apple.com/