about downgrade macOS

If you don't have another Mac, you'll need a 16 GB flash drive and a strong Internet connection. You'll be able to downgrade macOS to Big Sur, but the firmware and the other low-level components will still be on the version included with macOS Monterey. Also, you'll need to reinstall all of your apps after the downgrade.

Preparation

1. Download the macOS Big Sur installer from here: How to get old versions of macOS - Apple Support
2. If the Install macOS Big Sur app opens, quit it.
3. Connect the 16 GB flash drive.
4. Open Disk Utility, located in Applications -> Utilities.
5. In Disk Utility, select View -> Show All Devices.
6. Select the top level of the flash drive and click Erase. Choose these options, then click Erase:
   1. Name: MyVolume
   2. Format: Mac OS Extended (Journaled)
   3. Scheme: GUID Partition Table/Map
7. When the erase is finished, quit Disk Utility.
8. Create a bootable installer for macOS Big Sur: How to create a bootable installer for macOS - Apple Support
9. When finished, eject the flash drive. It now contains a copy of the macOS Big Sur installer.

Erase and downgrade

1. Start up from macOS Recovery.
2. If you're prompted to enter an administrator password, skip to step 5.
3. At the top of the screen, select Utilities -> Terminal.
4. Type this command and hit Enter (Return): resetpassword
5. The Recovery Assistant app should open. At the top of the screen, select Recovery Assistant -> Erase Mac.
6. Follow the onscreen prompts to erase your Mac.
7. Your Mac should restart and prompt you to activate it. If required, enter your Apple ID and password when prompted.
8. When the list of utilities appears, shut down your Mac.
9. Connect the bootable installer, then press and hold down the power button (Touch ID) until Startup Options begins loading.
10. At the boot picker screen, select Install macOS Big Sur and click Continue.
11. If prompted to activate your Mac again, do so to continue.
12. The macOS Big Sur installer should automatically open. Follow the onscreen instructions to reinstall macOS Big Sur.
13. When the Setup Assistant appears (multicoloured background), it is safe to remove the bootable installer.

Restore your data

In the Setup Assistant, choose not to migrate any data. When the Setup Assistant is completed, you should be at the desktop screen.

From here, you can transfer the files you saved on your external drive, and reinstall your apps from either the App Store or the developer's website.

Hi lyeng2004,

Did you make a Time Machine backup before you upgraded to macOS Monterey?

In macOS Big Sur, Time Machine doesn't back up the operating system, but it makes it a lot easier to restore your data in cases like this.

Regardless of whether you made a backup or not, it is possible to downgrade. That said, it will be considerably more difficult to preserve your data without a previous backup.

You would need to transfer all of your files to an external drive, along with any other data that you want to preserve. Make sure to look in the Documents, Desktop, and Pictures folders, as well as other places.

Before I post the steps to downgrade and erase your Mac, do you have another Mac (running macOS Catalina or later) available? This affects how the downgrade should be done.

Are you sure it's worth downgrading macOS? Which apps or files in particular don't work? It might be easier to identify the issues you're experiencing and solve those instead of trying to downgrade macOS.