Can I change a brand new, unlocked, not opened iPhone XS for a iPhone 11?

If you are still within the 14 return period, return it to the store where you purchased it for a full refund. Then purchase an iPhone 11 when they are released.

Are you within the 14 day return window? If you are return it now, don't wait. Then do as kb1951 suggests and buy the iPhone 11 when it's available for purchase. Right now, all you can do is pre-order and if you haven't yet, millions of people have likely already pre-ordered, so stocks will most likely be low on launch day (September 20th).

You have 14 days from the data of purchase to return it.

Syllvia wrote:

I am still in unated states

OK. But was it purchased at an Apple Store within 14 days. That's the only way you would be able to return it to an Apple Store for a refund. Even if unopened, it's not worth anything in exchange outside of that period,, unless you're talking about the trade-in program where the trade-in value is severely diminished.

You have 14 days from the purchase date to return your iPhone to Apple.

Are you no longer in the US? If you are now outside the US, in addition to having 14 days to return the phone, it would have to be returned in the country of purchase.