How update my iPhone 6 plus

An iPhone 6 cannot be updated past iOS 12.

An iPhone 6s—as indicated in your footer here—can be updated to iOS 15, but cannot be updated to the upcoming iOS 16 arriving later this year,

If you’re using an iPhone 6s and stuck at iOS 14 as is listed in your footer, then use a computer to update, as is described in part of this article:

… Update your iPhone, iPad, or iPod touch - Apple Support