How can I downgrade from iOS 26?

Here you go. If you did not previously make a backup of iOS 18.5, then all your data on your device will be lost. You will need a computer to remove the beta.

How to uninstall iOS or iPadOS beta software - Apple Support

You can downgrade to iOS 18, but unless you made a special "Archive Backup" of your phone on your Mac while it was still using iOS 18....and before you installed iOS 26.......you are going to lose all of the data on the phone when it reverts back to iOS 18.

An example of an Archive Backup would look like this:

Try taking a moment to actually read the information provided to you when you joined the developers beta.