How to update iPhone 8 to iOS 15.5

See how here --> Update iOS on iPhone - Apple Support

Note: If you have skipped a LOT of updates (which it appears you have), you may need to use your computer to update your iPhone. The following links will help depending on whether your computer is Windows or Mac:

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

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

You can't update an iPhone 8 to iOS 15.5 but an iPhone 8 can be updated to iOS 16.7.13. iOS 15.5 hasn't been signed by Apple in quite some time.

Apple stopped signing it and other intermediate OSes. To install the latest release, use the Software Update function in Settings or a computer.

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