How can I go back from beta 26 to iso 18 directly without deleting files?

Apple Beta — Restore Shipping Software

To use a previously-released OS version, you can erase your device, then restore from the backup that you created before you installed the beta.

There is no way to do what you want.

Apple made this clear when you went to the beta site and followed the directions to enroll in the beta program and install the beta software.

Stick it out for a few more weeks.

You don't.

Try actually reading the information provided when you joined the beta program.

Unfortunately, your question assumes that you can "go back". Can you go back without losing all the data on the phone?

Yes and No.

Yes, if you made a special "Archive" backup on your Mac or PC.....before....you installed the iOS 26 software.

No, if you didn't. In that case, it would be much better to stick it out using iOS 26 until the official release version appears.

Next time......you might remember that Apple strongly advises to only install beta software on a spare phone, not your "everyday" phone that you need to be dependable and reliable.