Looking to "downgrade" device

Yes, you can trade in after your trade in point. If you have the 24 month Next plan, you can trade in after 12 payments. If you have the 30 month Next plan, you can trade in after 24 payments. The credit you get for the trade in is the remaining payments you don't have to make. You will still pay full price for whatever new phone you buy. You are better off just paying off the phone, get it unlocked and sell it yourself.

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@Devonblackbriar wrote:
I purchased an iPhone 6s Plus through at&t next a few months ago. I've found that the device is too large for comfortable use and wish I had gotten a smaller phone. Is there any way to trade my 6s Plus for a smaller iPhone (even if it is a bit older) without paying the remainder of the at&t next contract? The size of the phone has become prohibitive and if I can't manage it within my current plan I may have to start looking for other providers.

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You really needed to trade it in the first 14 days. 

You need to pay it down enough to trade it in. Which next plan are you on?

Or sell it. (Which is usually more cost effective for the iPhones anyways).

It's a pretty big iPhone, I got the iPhone 7 PLUS last fall, if I didn't have the Apple Watch, I think I'd be pretty unhappy with the size (I don't need to pull out the iPhone for a lot of functions since I have the watch). I think I'm going to regret the choice come summer and shorts...

Yeah.  The option that might work best if you can't pay full price is go to Apple and buy an unlocked phone with Apples payment plan.  Then sell your ATT phone and use the money to pay off your Next installments.