How to install iOS 15.7 on my iPhone 6?

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.

If you have an iPhone 6, you cant't update to iOS 15. This doesn't mean that you can't use your iPhone 6 anymore. An iPhone 6s can be updated to iOS 15

You can’t. An iPhone 6 (no ‘s’) cannot run iOS 13 or higher. The most recent version of iOS 12 is the end of the line.