Unable to update iPhone 7 Plus

See If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support. If you're trying to update using iTunes try over the air instead, or vice versa.

Note if you're getting a -4000 error when updating from iTunes you need to remove your device passcode, update, then enable again afterwards.

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Try the advice in If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support.

Try updating using iTunes, or Finder if you use macOS 10.15 Catalina or later.

Or contact Official Apple Support for assistance.

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