How do I update my iPhone 8?

You can update an iPhone 8 to the latest release of iOS 16 – currently iOS 16.7.10.

You cannot update an iPhone 8 to iOS 17 or higher.

Update your iPhone or iPad - Apple Support

Updating iOS: common issues and solutions - Apple Community

About iOS 16 Updates - Apple Support

Go to Settings > General > Software Update, and it will tell you if your iPhone is up to date. The current version that I believe the hardware on the phone will support is iOS 16.7.10.

If what you are really asking is can your iPhone 8 handle iOS 17 or iOS 18......the answer would be no.

You would need to have an iPhone XR or higher model to handle iOS 18. (iOS 17 is no longer available).