What can I do to get my iPhone 6 to update to iOS 14.0?

There is nothing you can do to achieve that. 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.

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

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

You cannot pick and choose an iOS version. What you get is Settings > General > Software update > [All options are available here]

If iOS updates are no longer being released for your device, it typically means that the device has reached its end of life in terms of software support from Apple. In such cases, there are a few options you can consider:

1. Although major iOS updates may no longer be available, Apple may continue to release security updates for older devices. Ensure that your device is running the latest available version of iOS for its generation to maintain the highest level of security.
2. As iOS versions advance, some newer apps may not be compatible with older iOS versions. You may need to rely on older versions of apps or find alternative apps that are compatible with your device's iOS version.
   1. Ask the App developer to release the app for lower iOS versions also. Netflix & Amazon has done it.
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3. If your device is no longer receiving software updates, it may be a good time to consider upgrading to a newer device that is supported by the latest iOS updates. Upgrading to a newer device will ensure that you continue to receive the latest features, security updates, and app compatibility. Selection of a mobile model/brand is driv… - Apple Community

Remember to regularly back up your data to avoid losing any important information during the transition to a new device.

It's worth noting that the availability of updates and options may vary depending on the specific device model and its hardware capabilities.