I wanted to get back to iOS 18

Rolling-back to an earlier version of iOS/iPadOS is not possible.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.

If Apps no longer work properly following update to a more recent version of the OS, you’ll need to seek assistance from the related App Developer - as only they can resolve issues with their App.

How to contact an app developer - Apple Support

Perhaps consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would undermine the security of your device and your personal data. 

Thank you for reaching out. 🤗

According to Apple:

After a software update is installed for iOS, iPadOS, tvOS, watchOS and visionOS, it cannot be downgraded to the previous version.

Thank you! 👋🏼😉

Smiliñ 😎 Brian

Rather than tell us how you would like to solve an issue (using an impossible method, and, even if it were possible, would be undesirable), you should tell is what the issue is you’re trying to solve.

We may well be able to suggest a reasonable method to solve it.

You don't. Downgrading the firmware has never been supported on any OS version.