How can I update my iOS?

The ancient iPhone 6 Plus is only capable of updating as far as iOS 12.5.7. There are very few modern apps that continue to support this operating system.

The app developers at FaceBook, YouTube and other apps could allow their apps to run on older systems like iOS 12.5.7, but they have chosen instead to abandon users that have older iOS versions in favor of users with newer IOS Versions like iOS 16 and iOS 17.

If you want to run most modern apps, you will need to have an iPhone that is capable of at least iOS 16, and preferably, iOS 17.

An iPhone 8 will run iOS 16. An iPhone XR and higher models will run iOS 17.

This may help you --> Apps that require higher iOS versions than your device can be updated to... - Apple Community

An iPhone 5s, iPhone 6, or iPhone 6 Plus isn't capable of 13 or above; they require at least 2GB of RAM. Try downloading the applications to a newer unit on the same Apple ID first.

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