I want to update my iPhone to iOS 15.8

What model phone do you have?

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.7.8

• If you have an iPhone 8 or X, you can update to 16.7.1

• If you have an iPhone SE (2nd generation or later), or XR through 15, you can update to 17.1

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

How to update my iPhone

Update my iPhone 6 iOS 15

It is not possible to update an iPhone to iOS 15.

You need an iPhone 7 to update to iOS 15.

The iPhone 6 can update only as far as iOS 12.5.7.

This has already been answered in this thread. Look for the answers above.