Apple.com exchange

You have 14 days to do so. Thats Apple's return policy.

 

FWIW, if you read their official rules for refunds (link below), it says "Note: An iPhone purchased on the Apple Online Store is ineligible for return to an Apple Retail Store.". However, some folks have posted on here that they've been able to do that. In the event the store will switch you out, you're still dependent on the store having what you want in stock.

http://store.apple.com/us/help/returns_refund

 

I was in the same situation but decided to sell it on CL instead of returning it. Needed the expense on my credit card towards earning 40k miles. Found a buyer on CL same day for $120 above what i paid.

 

Not sure about exchanges since the 6+ is in hot demand right now.

But you should be able to go to a retail store and return the phone you bought online even though Apple's web site and online and phone reps will tell you that they need to be shipped back. I just returned the 6 I bought online to the Grand Central store here in NYC. Refund will take a few days but my upgrade eligibility with AT&T was reinstated immediately. So I'm free to buy another one now at the subsidized price if I want to.

To do the full exchange you'll probably need to first return then wait in the walk-in line to take the chance they'll have the one you want. That's what I was told by the guy who took care of my return, but may depend on the store you go to.