How can I update my iPhone 8 to iOS 18?

An iPhone 8 can't be updated beyond 16.7.11, it doesn't have the hardware required to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

• If you have an iPhone 8 or X, you can update to 16.7.11

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.4

If you want to get iOS 18, you will need to buy a new iPhone that is compatible with it.

iPhone models compatible with iOS 18 - Apple Support

I would suggest an iPhone 15- or 16-series phone that is likely to have years of updates ahead of it. If you also want Apple Intelligence, the only new iPhones that Apple sells which support it are the 16-series phones.

If you buy a new phone, you may find the following Support articles useful in helping you to transfer your data to the new phone, and in properly resetting the old phone (if you are not planning on keeping it).

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Finally, if you are tempted to buy a used iPhone to save money, please read

The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community

so that you will know, e.g., what to look out for to avoid buying an Activation-Locked and possibly-stolen "brick."

You can’t. Your iPhone 8 lacks the hardware and power to run iOS 18. It also has already reached the end of the road for its iOS updates and can’t go past 16.7.11. As a matter of fact, it got a security update the other day, but it still won’t ever go to 18. iOS 18 is supported on the XR and later.