iPhone 6 new update not available

These are the iOS options based on iPhone Model --> You need to correctly Identify your iPhone model - Apple Support

• iOS 16.1--> SE (2nd and 3rd Gen), 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max, iPhone 13, 13 Mini, 13 Pro, 13 Pro Max, 14, 14 Plus, 14 Pro and 14 Pro Max
• iOS 15.7.1 —> 6S / 6S Plus, SE (1st Gen), 7 / 7 Plus,
• iOS 12.5.6 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, 
• iOS 10.3.4 --> iPhone 5
• iOS 10.3.3 --> iPhone 5C
• iOS 9.3.6 --> iPhone 4S
• iOS 7.1.2 --> iPhone 4

Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

If you want to run those programs, you will need a newer phone that can run a higher version of iOS. Many programs will no longer run on iOS 12.5.6 they require features that are found in the more recent operating systems.