Can I update my iPhone 8 Plus to iOS 18?

You cannot pick and choose an iOS version. What you get is Settings > General > Software update > [All options are available here]

You need to correctly Identify your iPhone model - Apple Support

These are the iOS options based on an iPhone Model

(Updated Jan 4, 2025 12:05 PM)

• iOS 18.3.1 —> iPhone models compatible with iOS 18
• iOS 16.7.10 —> iOS 16 and iPadOS 16 are compatible with these devices
  • Chipset A11 - 8 (2GB RAM) / 8 Plus (3GB RAM) / X (3GB RAM)
• iOS 15.8.3 —> iOS 15 is compatible with these devices.
  • Chipset A9 - 6S (2GB RAM) / 6S Plus (2GB RAM) / SE (1st Gen) (2GB RAM)
  • Chipset A10 - 7 (2GB RAM) / 7 Plus (3GB RAM)
• iOS 12.5.7 —>
  • Chipset A7 - iPhone 5S (1GB RAM)
  • Chipset A8 - iPhone 6/6 Plus (1GB RAM)
• iOS 10.3.4 —>
  • Chipset A6 - iPhone 5 (1GB RAM)
• iOS 10.3.3 —>
  • Chipset A6 - iPhone 5C (1GB RAM)
• iOS 9.3.6 —> iPhone 4S (512MB RAM) - Chipset A5
• iOS 7.1.2 —> iPhone 4 (512MB RAM)- Chipset A4

Know how to...

• Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
• Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If the above is not helping...

Probably you are seeking to update becuase --> New Apps that require higher iOS versions than your device can be updated to... - Apple Community

No, that will not happen. Your iPhone 8 Plus is at the end of the road for the latest iOS software updates and cannot be updated past iOS 16.7.10. iOS 18 requires an iPhone XR or later, so you will need to purchase a newer iPhone to run that.

Probably you are seeking to update becuase --> New Apps that require higher iOS versions than your device can be updated to... - Apple Community

Please lunch iPhone 8PLUS 18 update

replay me you lunch or not

The iPhone 8 Plus will never update to iOS 18....because it is not qualified to do so.

If you want to update to iOS 18, you will need to have an iPhone XR or higher model.