How do I update my iPhone to iOS 15.8?

Use a computer or the Software Update function in Settings to install 15.8.4. If you encounter a specific issue other than the one listed below, describe it in this thread.

Apple is no longer signing 15.8.3, so it's no longer possible to update to that exact version.

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Make sure that the phone is connected to a strong Wi-Fi signal and the battery is at 70%+

Tap Settings > General > Software Update

Update your iPhone or iPad - Apple Support

The iPhone will update to iOS 15.8.4.....not 15.8.3

If you are still having problems, you will need a Mac or PC to run the update.

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

There isn’t much detail provided, but to update your phone software, just go to settings -> general -> software update