Can I return my iPhone 6 for a 6s?

I had the iPhone 6 for a long time and wanted to get the iPhone 6s and I'm wondering if I can return my iPhone 6 for the iPhone 6s

Not if you had it for a long time. You can do an upgrade and pay the money to do so. Sometimes, depending on your carrier, you can get money credit for the 6 to use for a 6s.

Some shops allowed within seven days with original packing.otherwise trade in.

Best option is to trade it in

You if you pay more money anyone will gladly take your business.

If you trade-in your iPhone 6, you'll get discount on the iPhone 6S you're about to get.

Nope, the return period was over after 14 days from the retailer that you got the phone from. So they are not going to take a used phone and replace it with a new phone.

Now if the carrier you are with has a trade in program, and your phone is paid for, you might ask if the phone you have is eligible for a trade up to a newer phone.

Trade ins are offered yes.