Why can’t I update my iOS to the latest version even after backing up my iCloud

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device. For an iPhone 6 that is iOS 12.5.7.

This support article has instructions about updating your iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software: Update iOS on iPhone - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date and won't offer to install anything. Apple indicates ( Unable to update my iPhone 6 Plus from 12… - Apple Community ) I need to point out the obvious and state that if you want to run a higher system version than your current device will run, that you need to buy a newer device.

If your device won't update, make sure you are following all the conditions for the method you are trying to use.

If your Apple mobile device won't update, read this support article for a list of things to try --> If your iPhone or iPad won't update - Apple Support

Another, less common reason for not being able to update is if you were using a beta version of software. Refer to How to uninstall iOS or iPadOS beta software - Apple Support

Because your iPhone model 6 cannot support iOS versions beyond 12.5.7, as per this compatibility list: