Check whether Apple is still signing iOS 18 for your iPhone model
Back up your iPhone data before downgrading
Download the correct iOS 18 IPSW file for your device
Use a Mac with Finder or a Windows PC with iTunes
Connect your iPhone to the computer with a cable
Put your iPhone into recovery mode
In Finder or iTunes, choose Restore iPhone while holding Option on Mac or Shift on Windows if selecting the IPSW file
Select the iOS 18 IPSW file and start the restore
Wait for the restore to finish and let the iPhone restart
Set up the iPhone again and restore your backup if compatible
If iOS 18 is no longer signed, you cannot downgrade through normal methods
If you need help, contact Apple Support or use a trusted repair service