Can I trade my iPhone 6 for an iPhone 7 when it comes out?

I want to buy the 7 when they release it on September, but my contract ends next year on February bc AT&T sucks.

You can buy the iPhone 7 and replace the iPhone 6 that's on that line, but you won't be able to leave AT&T before February without an ETF. Another option you could look at is see if another carrier is willing to buy out your ETF.