How do I update iPhone iOS 15.8.3 to iOS 16?

Your tag line says that you have an iPhone 7 Plus.

The iPhone 7 Plus originally shipped with iOS 10 – and you can update it to the latest release of iOS 15, currently iOS 15.8.3. You cannot update an iPhone 7 Plus to iOS 16 or higher. If you need something more recent than iOS 15, it's time to get a new iPhone.

You can’t; your 8 year old iPhone has the highest version available for that model. If you need a more recent iOS version you will have to get a new iPhone.

Start here: Update your iPhone or iPad - Apple Support. You don’t say what phone you have and are asking about very old iOS versions so you may not be able to get the newest iOS version.

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