how do I go back from montrey OS to big sur ?

I would follow the Previous Contributor advise First.

Though there ways and methods to Downgrade , do understand the consequences and ramification Before Attempting.

The link below is one method

How to Downgrade MacOS Monterey to macOS Big Sur

And then there is >>

" Scorched Earth Method  " that >> Only works on Intel Based Apple Computer <<

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

 About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map.  

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

Not easily, and — depending on the macOS version originally installed on that Mac — perhaps not possible at all. The solution requires completely erasing the Mac, installing its originally installed operating system, upgrading to Big Sur, followed by installing all your required apps and restoring documents that have not yet been upgraded for Monterey.

You would be far better off describing the problems you are experiencing. They may be fixable. What is not fixable are programs that require Big Sur and do not run or have not been upgraded to run in Monterey.