How to Downgrade iOS 18 to iOS 17?

This is the method provided by Apple. You would need a backup from iOS 17 to restore your data, otherwise everything will be wiped. This was recommended when you chose to install the beta:

Apple Beta — Restore Shipping Software

The beta page specifically provides you these important points that should not be ignored:

• Always back up your iPhone, iPad or Mac before installing the beta. Since Apple TV, HomePod mini, and HomePod (2nd generation) data and purchases are stored in the cloud, there’s no need to back up your devices. To use a previously-released version of iOS, iPadOS, or macOS, you can restore your device from the backup that you created before installing the beta. Please note Apple Watch cannot be restored to previously-released OS versions once the beta is installed.
• Beta software may contain errors or inaccuracies and may not function as well as commercially released software. We encourage you to submit feedback when you encounter these issues. If you need further assistance, please note that you must restore to previously-released software prior to contacting AppleCare.

Apple Beta — FAQ