How to upgrade iOS
How do I upgrade iOS on my iPhone 6
You need to correctly Identify your iPhone model - Apple Support
- iOS 14.7.1 —> 6S / 6S Plus, SE (1st Gen and 2nd Gen), 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max
- iOS 12.5.4 —> Final release supported on iPhone 5S, iPhone 6/6 Plus,
Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support