My 'new' iPhone 6+ has iOS 8. I want to update to iOS 10. How?

My iPhone 6+ was just replaced by Apple. Thank you Apple!

The new iPhone 6+ has iOS 8.1.3 on it. I guess it has been sitting around in their warehouse for awhile.

I want to update it to iOS 10 because iOS 11 is laggy on 6+. I cannot find how to do that. The only option is "the latest version of the operating system."