How do I update my iPhone 7 Plus from iOS 13.7 to iOS 15.3.1?

It's no longer possible to update an unit to 15.3.1; Apple stopped signing it. To install 15.8.4, use a computer or the Software Update function in Settings; if you encounter a specific issue, describe it.

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Update your iPhone, iPad, or iPod touch

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.4

• If you have an iPhone 8 or X, you can update to 16.7.11

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.4.1

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support