Hi I bought Apple Watch but band is little tight can we change band size

I think it depends on the band me of course if you are still in the exchange/return period. When I wanted to exchange the magnetic link one that I got with my series 9 I was told I would have to exchange the whole watch because the only watches you can exchange just the band are the Solo Loop and Braided Solo Loop. No clue why though. Contact Apple Support (especially if you ordered your watch versus in store purchase) to find out for sure though.