If that won't work because your device is really full, you can upgrade via a Factory Reset. Before doing that, first go to Settings, your name, iCloud, See All, and make sure ALL the switches are green (except iCloud Mail if you don't have an @iCloud.com email address). Then, back out (blue < arrow in the top left), and go to iCloud Backup and Back Up Now. Once that's done, you can back out of that, and go to Settings, General, and down at the bottom, Transfer or Reset iPhone/iPad. Make sure you know your Apple ID password before proceeding! Tap on Erase All Content and Settings and follow the prompts, and when your device has finished being Factory Reset, choose the Quick Start option and then Restore from iCloud Backup. and choose the backup you just did a few minutes before. It'll prompt you to install 18.0.1 before restoring all your Settings, and apps and content.