updating Mac OS

ishu194 Said:

"updating Mac OS: I updated my Mac to Monterey then it was not stable. so I decided to downgrade my Mac but I downgrade to Mojave now , I am not able to upgrade to Catalina or big sur as it get stuck at checking null [...]Also I tried safe mode"

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Three Thoughts:

A. Try Ethernet:

Ethernet is hardwired. So, a connection would be more reliable. For a Mac that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple.

B. Try a New User:

Try creating a new Administrator user, seeing if you can get this to work.

C. Try Force Quit:

Use this User Tip: [command+tab] Use it to Access your Mac when Something is Not Responding - User Tip

In Depth:

First, you need to exit the apps and get to Finder (the Smileyface):  Once in Finder, use Force Quit...

Use Force Quit for Each App:

1. Go to: Apple menu
2. Select: Force Quit...
3. Select: an app
4. Click: the Force Quit... button
5. Click: the Force Quit... Button.
6. Perform seeps 3-6 for all listed apps
7. Restart your Mac

ishu194 Said:

"updating Mac OS: I tried B and C but not able to update my Mac still getting checking null"

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Use a Different Mac:

Try downloading a USB Bootable Installer on a different Mac. Have none? Then as someone else (i.e. friend or family member) if you may use theirs at the moment (with their permission, of course).

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

Tips Big Sir Upgrade Released Nov 2020

Tips Monterey Upgrade  Released Oct 25, 2021

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

Check in Applications folder.

This error usually indicates the Full Installer is already in the Applications folder. Drag and Drop to Trash, Empty Trash and re-download the Full Installer using Only Safari.

Are these two posting related ? Which one is the present condition of the computer.

https://discussions.apple.com/thread/253360365

Please re-read the above posting.

The Link is provided and here it is again How to get old versions of macOS

Sorry one is having so many issues.

Think, at this point, it might be better if you take the machine to a Qualified and Certified Apple Service Centre have them put the computer right.