How do I update my iPhone 8 iOS?

The iPhone 8 is not compatible with iOS 17 or higher.

If this is not the issue, please provide relevant information.

Apple does no longer provide software updates to devices that came previous to the iPhone XS/XR. You cannot update your iPhone anymore, it will not be possible. As you also cannot install a different operating system like on android devices (which you have to because the support usually only lasts a few years, in comparison to the 6 years your iPhone 8 survived), you will not have any option except from getting a new phone or continue to use your iPhone without an update, which is not a good idea at all. I’m sorry for you.