iPhone on ios 12.4.9 says its up to date.

iPhone 5s, iPhone 6, and iPhone 6 Plus are not compatible with iOS 13 or iOS 14. iOS 12.4.9 is the latest version available for iPhone 6 Plus.

iOS 14 and iPadOS are compatible with these devices - Apple Support

Apple security updates - Apple Support

Any Apple device older than September 2015 or October 2014 will be able to update to the latest software The reason why is because the CPUs will now be ample to title of iOS 13 and iOS 14 S capabilities, the devices that stand on iOS 12 with a 3013 iPhone fathers, the 2014 iPhone 6 and Orston 6+, the 2013 iPad Air and 2013 iPad mini, 13,014 iPad mini, and the iPod touch six