iPhone 6 is not updating

Hi - your iPhone 6 was released by apple in 2014. The SE in 2016 with an update in 2017.

* iOS 14.4 —> 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 12.5.1 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, iPad Air, iPad Mini 2, iPad Mini 3 and iPod Touch (6th generation)

* iOS 10.3.3 —> Final release supported on iPhone 5C and iPad (4th generation)

* iOS 10.3.4 —> Final release supported on iPhone 5 and iPad (4th generation) Wi-Fi + Cellular

* iOS 9.3.6 —> Final release supported on iPhone 4S, iPad 2 Wi-Fi + Cellular, iPad (3rd generation) Wi-Fi + Cellular, and iPad Mini (1st generation) Wi-Fi + Cellular

* iOS 7.1.2  —> Final release supported on iPhone 4.

* iOS 6.1.6 —> iPhone 3GS and iPod Touch (4th generation) only

* iOS 4.2.1 —> Final release supported on iPhone 3G and iPod Touch (2nd generation)

* iOS 6.1.5 —> Final release supported on iPhone 3GS, and iPod Touch (4th generation)

* iOS 3.1.3 —> Final release supported on iPhone (1st generation) and iPod Touch (1st generation)

How to find your phone model:

https://support.apple.com/en-in/HT201296

Your iPhone 6 is not compatible with iOS 13 or 14.