how to deleat an upgrade

First you need a time machine backup of your data. Because this will involve erasing you hard drive and then using internet recovery to install the OS that came with you computer. At which time you can then use the App Store to upgrade to what ever OS you wanted to go back to. And then restore your data with time machine. One other way is to download and then create a bootable flash drive for the OS that you want installed so as not to go back al the way to you original OS. But depending on the OS that came with your computer and the OS that you want installed, there still could be intermediate steps. https://discussions.apple.com/thread/252595805 Her is where you can get older Mac OS. How to get old versions of macOS - Apple Support

If one is absolutely certain this is the best course of action ?

Do suggest following the good advise from previous posting and work through the issues before doing a " Scorched Earth " approach.

Only after every possible remedy has been exhausted then and only then >>>

Below will WIPE ALL DATA and there is No Recovery - Period.

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

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.

Once that is done read on for preparing the Destination computer 

>> Only works on Intel Based Apple Computers.

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.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the first window that comes up so that it can get additional information from the Console and log files for the report:

Also click the About… vurron and read the info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

like this:

Depending on your level of expertise, it may be easier to try to resolve your issues with Monterey than revert to Big Sur. In particular, Monterey Version 12.2.1 had resolved several issues.

For Messages, try checking Enable Messages in iCloud in Messages Preferences on your Mac. Also, make sure Messages is enabled as an iCloud app on your iPhone.