Help upgrade my iOS to iOS 13.4

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on 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. An iPhone 6 (or 6 Plus) will only update as far as iOS 12.5.7. If you want to run a higher system version than your current device will support, you need to buy a newer device.

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

That's not possible. The iPhone 6 can not run anything higher than iOS 12.