Back up your iPhone data first
Check whether iOS 18 is still being signed by Apple
Download the correct iOS 18 IPSW file for your iPhone model
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
Open Finder or iTunes
On Mac, hold Option and click Restore iPhone
On Windows, hold Shift and click Restore iPhone
Select the iOS 18 IPSW file
Confirm the restore and wait for the process to finish
Set up your iPhone after the restore
Restore your backup if it was made on iOS 18 or earlier
If iOS 18 is no longer signed, you cannot downgrade using Apple’s standard restore method