Apple series 3 38 mm
Can I go to any Apple store and tell them I want the 42 mm series 3 watch instead can I trade in my 38 mm and would I have to pay anything
Apple Watch Series 3, 6
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If you bought the 38mm less than 14 days ago, you can return it for a full refund: https://www.apple.com/shop/help/returns_refund
If its been more than 14 days, you can trade it in, but will get less value for it: Apple Trade In - Apple
You will have to pay a difference regardless, as the 42mm version is in all cases more expensive than the 38mm version.