iPhone 6 and iPhone 6 Plus cannot update to iOS 17
Hello and welcome to the thread.
You have an iPhone 6 or iPhone 6 Plus, and wish to update to iOS 15.
Nope. Not supported.
iOS 12 is the end of the line for iPhone 6S and iPhone 6 Plus; to iOS 12.5.7, currently.
You’ll need a newer (used, refurb) or new iPhone.
An iPhone 6S or iPhone 6S Plus can update to iOS 15; to iOS 15.8.2, currently.
Any recent iPhone will be substantially more capable than an iPhone 6, 6 Plus, 6S, or 6S Plus, as well.
[Re-Titled by Moderator]