can i trade my ipone 6s to iphone 7

Hi

In some countries (including the US), Apple offers a trade-in program for iPhone. This offers instant in-store credit towards the cost of a new iPhone (if trading in at an Apple Store) or a Gift Card (if trading in via mail) when trading in an eligible iPhone.

In the US, for example, the estimated trade-in value of an iPhone 6s is currently $225 (terms apply):

http://www.apple.com/iphone/trade-up/

When buying an iPhone 7 (there is currently no 7s), you may also find the iPhone Upgrade Program (also available in selected countries) of interest:

http://www.apple.com/shop/iphone/iphone-upgrade-program

Another option is to sell your iPhone 6s privately and put the proceeds towards your new purchase. If doing so, be sure to follow the steps here:

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

Yes but you will get at most $225, might be best to sell it on your own. See this article for details.

http://www.apple.com/iphone/trade-up/