Hi
Depending on your location, Apple may offer instant in-store credit when trading in eligible iPhone models or an Apple Store Gift Card when trading in your devices by mail. Note that, at present, the online tool for checking trade-in value does not include the iPhone 6s.
To learn more and for examples of estimated trade-in values in the US, for example:
To complete an online evaluation for eligible models (US):
You may also wish to consider the option of selling your various iPhone models privately and putting the proceeds towards your new purchase.