How can I return Watch band?

Also Note: If you are within the return window and you own Apple Watch Ultra and you want to return just the band, you can't. When you buy an Apple Watch and Band, they are considered ONE item. You'd have to return the watch AND band together in the original packages with all accessories included. You could then buy the watch and band combination you'd prefer.

If you are outside the return window, buy yourself a band you'd rather wear and you can do that from Apple or Amazon or BestBuy or from numbers of 3rd party companies. Nomad is a company I have purchased several bands for my Ultra and they are top notch. I have loads of high quality aftermarket bands for my Apple Watch Ultra.

in some countries there is always a 14 days window for a full refund

try reading

Legal - Hardware Warranties - Apple

and

Returns & Refunds - Shopping Help - Apple