There is no option for an iOS 14 update as it says my software is up to date on the iOS 12

iOS 13 and higher requires 2GB RAM or more and a 64bit platform. iPhone 6/6+/5s has 1GB RAM

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

• iOS 15.0.2 —> 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, iPhone 13, 13 Mini, 13 Pro, 13 Pro Max
• iOS 12.5.5 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, 

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

That is correct. You have an iPhone 6 and your iPhone is running iOS version 12.x.x. That is the highest version of iOS that you can run on an iPhone 6. You will need a newer iPhone model to run iOS versions 14/15.

Axel F.