How much would it cost to upgrade an iPhone 5 to 5s or 6?

I have an iPhone 5 that has problems of shutting off and remaining that way for hours every few been happening more frequently in the past few other than that it looks perfectly fine. It's an original from when the 5 first came out and has never been replaced or fixed. If I passed it in to at&t, how much would the upgrade cost?

I would check your carriers site.