How do I update my iPhone 8plus

Which update are you referring to? 

1. iOS Update? Which Version? Update your iPhone, iPad, or iPod touch - Apple Support
2. Carrier Update? Contact Carrier. Update your carrier settings on your iPhone or iPad - Apple Support
3. Update your payment method: If you need to change or update your Apple ID payment ...
4. App Update? Which App? Contact App Developer? How to contact an app developer - Apple Support
5. Update your Apple Card account information: Update your Apple Card account information
6. Apple News Update? --> Read the latest headlines in the Apple News app - Apple ...

iPhone 8 Plus cannot update to iOS 17, if that is what you might be asking about.