Update new iOS versoin iphone 6

You need to correctly Identify your iPhone model - Apple Support

• iOS 14.7 —> 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

iPhone 6 and 5S have only 1GB RAM which isn't good enough for iOS 13 and above.

Apple Support Community Forum

This is Apple Support Community forum, for a user to user interaction for information & idea sharing between users, if need be, extend support on technical knowhow in handling Apple products from a more experienced fellow user, This is not the right forum to discuss with Apple Inc.