How can I update my iPhone 6 Plus?

Hello,

An iPhone 6 (or 6 Plus) will only update as far as iOS 12.5.7. To ensure you have at least that, you can try updating your device. 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. If you want to run a higher system version than your current device will support, you need to buy a newer device.

Even though your profile information says you are running iOS 12, I do not automatically assume it is fully updated to the current version. 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

Your iPhone 6 Plus can be updated to iOS 12.5.7, no further.