I want to update my iPhone 6

An iPhone 5s, 6, and iPhone 6 Plus through hardware limitation,

if not already there, can only see an update to iOS 12.5.6 at best.

[..Later build models, 6s, 6sPlus, SE (1st gen), 7, & 7Plus; could see

update to iOS 15.7.2 ~with 'perhaps' a security update to iOS 15.7.x.]

Update iOS on iPhone - Apple Support (variable by build hardware)