How can I update version my iPhone 7 Plus

An iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, or iPhone SE 1 isn't capable of 16 or above; they require an A11 or better CPU. To install 15.8.1, use a computer or the Software Update function in Settings.

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You might be considering updating your iOS to a newer version, as some apps may require a higher iOS version than 15.8.1 to install. --> Apps that require higher iOS versions than your device can be updated to... - Apple Community

ya125 Said:

"wanna update my iPhone 7 plus"

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What's the current iOS that is installed?

About Updating an iPhone:

1. Back Up First:

IMPORTANT: If you have an option to update, however, then be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update

then...

2. Proceed with the Update:

Use my User Tip:  Update your iPhone Using a Computer - User Tip