unable to find beta version update toggle

That's because the beta for the upcoming iOS 26 is only for developers at this time. Public beta testing won't happen for several weeks. And if you feel you want to be a beta tester, be prepared for many things not working. Apple always advises to NEVER install beta software on your primary device. If you have any issues, you will NOT be allowed to ask for help here.

The public beta is not available. Currently you must be a Developer and have a Developer Account. Even then you would NEVER want to install it on a daily device and it should only be tested on a spare device where you do not care about data loss. To revert back, you will also need a computer, otherwise you will be stuck with software that may not even work correctly as it clearly is not finished and those bugs are expected.

That might actually be a good thing, as it appears that a significant number of users who have downloaded the "update" are now asking how to revert back to iOS 18.

Just one of many examples that we have seen today.........how to remove ios 26 beta - Apple Community

It would be a good idea for you to at least wait for the "Public Beta" to install the software on a spare phone.....NOT your regular phone.