How do I update my iPhone 7 Plus to iOS 18.2?

You don’t.

Your iPhone 7 is already running the most current iOS of which it is capable.

Newer iOSs require newer iPhones. iOS 18 requires an Xs/Xr/SE2 or newer.

Note: Simply renaming your phone as an “11” makes no difference.

The iPhone 7 Plus originally ran iOS 10. You can update it to the latest version of iOS 15. You cannot update it to iOS 16 or higher. If iOS 15 isn’t good enough, it is time to purchase a new phone.

This iPhone is too old to run anything higher then the latest version of iOS 15 I am afraid