How should I recylce my old IOS devices? And how is trade exchange made with Apple company?

Hey,

I have learnt of a beautiful news from Apple Store and I would like to recycle my Iphone 3GS, Iphone 6, Iphone 4 and Ipad 2.

If I want to get latest Iphone and Ipad, I'll need to give above Iphone 6 which disabled me from exchanging, and so I thought of recycling my old IOS devices.

I do not know exactly how things are done, so I hope someone can explain how it works with Apple company.

Thanks