Back up your iPhone
Check whether Apple is still signing iOS 18 for your device
Download the correct iOS 18 IPSW file for your iPhone
Install the latest version of Finder on Mac or iTunes on Windows
Connect your iPhone to your computer
Put your iPhone into recovery mode
On Mac, open Finder
On Windows, open iTunes
Select your iPhone
Hold Option on Mac or Shift on Windows and click Restore iPhone
Choose the iOS 18 IPSW file
Confirm the restore
Wait for the installation to finish
Set up your iPhone after the restore
Restore your backup if compatible with iOS 18