How do I update my iPhone 8 Plus to latest iOS version?

Unless you have not updated in some time, you already have the latest version for the iPhone 8 Plus.

Tap Settings > General > Software Update

If an update is available, it will appear

If you see a message that your iOS is up to date, you already have the latest iOS version.

The iPhone 8 Plus cannot run iOS 17, if that might be what you are asking about.

If your iPhone is eligible for an update, see the information in this support document to complete an update. Update your iPhone or iPad - Apple Support