How to downgrade iPadOS 26 to iPadOS 18 without losing data or a computer?

I hope you have a pre-existing backup of your essential data and files...

Restoring the production version of iPadOS will completely erase the iPad. Restoring iPadOS 18.x will also, unavoidably, require the use of either a Mac computer or Windows PC.

How to uninstall iOS or iPadOS beta software - Apple Support

While the restore process will fully erase the target iPad, if you have a an iTunes or iCloud backup that pre-exists installation of the beta, you can restore your existing backup:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Be aware that you can only restore an iCloud or iTunes backup to the same or later version of iPadOS than was used to create (or update) the backup. As such, any backup created/updated by the iPadOS 26 beta cannot be restored to an earlier version of iPadOS.

You'll need to erase and restore from the backup you may before installing. See How to uninstall iOS or iPadOS beta software - Apple Support