Upgrade

It doesn't really matter where you purchased your iPhone, but Apple is not taking in iPhone 6 in trade any longer. The earliest version iPhone is taking in trade is iPhone 6s.

See this link --> https://www.apple.com/shop/trade-in?afid=p238%7CLjomgr00-dc_mtid_1870765e38482_pcrid_78615220252087_pgrid_2959797630_&cid=aos-us-kwbi-brand--slid---product-

You can check Apple’s site for trade in value of your phone:

https://www.apple.com/shop/trade-in

You can sell your current phone to someone and use the money to buy a new phone.

Honestly, an iPhone 6 is not going to get you much.

It has very little value and you would be better off gifting it to a child or senior that needs a more modern device.