Cant Downgrade my Iphone 14 pro max

A little now as you've made a mess for yourself but read this --> To Beta or Not to Beta - That is the Ques… - Apple Community

Since you didn't read any of the documentation before installing not ready for public use software on your iPhone and most assuredly didn't make an Archive Backup using your computer, the advice Apple gave you to wait until it is officially released is the best advice at this point. But the link I posted does tell you how to restore your iPhone back to the current iOS. You'll just lose all your data doing so.

You were able to downgrade from iOS 18 to iOS 17 for a short period of time while Apple was still signing iOS 17. If you tried the same thing now, it would be impossible.

Downgrading from beta software is a completely different situation.

Somehow, you must have missed the very important instructions from Apple that you should only install beta software on a "spare" phone.

i even took it to Apple service centre, they told me to keep using it and wait for the official release,

That is excellent advice.

and they told me they can, but there could be risk to it

You will very likely lose all the data on your phone if you revert to iOS 18......unless.....you made a special Archive Backup of your phone on your Mac while it was still running iOS 18.....before you installed the beta software.

Example.......

Next time, it would really be best if you could pay very close attention to Apple's recommendations before you roll the dice on developmental untested software.

Seems you are committed to the iOS 26 beta and as Apple has already informed you, if the Apple-provided process for resetting and reverting is apparently not working:

How to uninstall iOS or iPadOS beta software - Apple Support

Betas are not without risk to devices and data, and all part of why running a beta on any device you need to work, with any third-party apps you need to work, and anyniCloud data you need available and consistent, is a bad idea.

manmeet241 wrote:

yeah for sure its not like i use anything apart from safari for manga and lite social media apps.

just wanted to downgrade to fix jitters inbetween in ios

Although, and respectfully, if that “not like I use anything…” usage were actually the case, this thread would not exist.

You can perform the reset and revert, or can log some bug reports for whatever issues you find, and can continue through with the iOS 26 beta.

You can't because you can't. Maybe you should have done 5 minutes of research before installing beta software.

Maybe next time, you will.

"Jitters", "bugs", "glitches" and "surprises" would be expected with any beta software.

That's why Apple advised you to only install the beta software on a "spare" phone.