How can I downgrade from iOS 26 to iOS 18.5 without a computer?

Downgrading Beta Software to Public iOS requires a computer. If you don’t have one, ask a friend or a neighbor to borrow theirs. If you still can’t, you can visit your nearest Apple Store’s Genius Bar or Apple Authorized Service Provider where they can help you.

How to uninstall iOS or iPadOS beta software - Apple Support

Well, you can get iOS 18 back by following the instructions to use a computer to do so.

But, the fact that you are telling us that you don't have a computer would indicate that you did not make an Archive Backup of your iPhone while it was still using iOS 18......before.....you installed iOS 26.

You would have needed a Mac to make an Archive Backup.

If you did not make an Archive Backup, you are going to lose the data on your iPhone when you restore it back to iOS 18. If you don't want to do that, you are going to have to stick it out with iOS 26 until the official release sometime this fall.

It is also unfortunately too late now to point out that Apple strongly recommends only installing iOS 26 on a "spare" phone......not your regular phone.

You should have read and understood the terms before participating as published:▷ Frequently Asked Questions.

Learn how to ⇢ downgrade here.

For further assistance:▷ Apple Developer Support.

Good luck! 👋🏼😉

You might get iOS 18.5 back, but if you didn't make an archived backup before installing the beta, you're probably in for data loss now. You cannot restore a phone running iOS 18.5 with a backup made using the iOS 26 beta.