how to downgrade to ios 18.5 from 26 beta

You must use a Mac or PC. There are no exceptions. If you don't have a Mac or PC, you will need to borrow one or take the phone to a service shop that has a Mac or PC.

How to uninstall iOS or iPadOS beta software - Apple Support

In addition to what Bob Timmons provided, since you don't have a computer, you must not have made an archive backup of your iPhone ON a computer before you installed iOS 26. That means you are going to lose all the data on your iPhone when you erase it to go back to iOS 18.5.