Exchanging MacBook Air 15-inch for 13-inch model.

How recently?

You have 14 days from the date of purchase from Apple or an authorized reseller to return it for a full refund, no questions asked, which you can then use to buy the 13" model. There is no direct trade as such however.

If you are past that time frame, then you will need to sell your MacBook Air privately and use the money to buy the 13" version.

Apple offers exchanges within 14 days in most countries.

If you bought it from another store, check their policy.

If you don't qualify in either case, sell it to someone else.

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If you find yourself outside the return window for this computer and need to sell it off before you buy your preferred Mac, please do the following before you sell it:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support  

If you don't, then you'll be selling someone a brick that they cannot properly configure and personalize for their own use. And I'm sure you don't want to be that person.