Topic: How to erase and install your Mac OS
ERASING YOUR MAC OPERATING SYSTEM
1. Turn on your Mac and immediately press and hold these two keys: Command (⌘) and R.
Release the keys when you see an Apple logo, spinning globe, or other startup screen.
You might be prompted to enter a password, such as a firmware password or the password of a user who is an administrator of this Mac. Enter the requested password to continue. Startup is complete when you see the utilities window.
2. Select Disk Utility
Choose View > Show All Devices from the menu bar in Disk Utility. The sidebar now shows your disks (devices) and any containers and volumes within them. The disk your Mac started up from is at the top of the list.
3. Select the disk that you want to erase.
4.Click Erase, then complete these items:
Name: Type the name that you want the disk to have after you erase it.
Format: Choose APFS or Mac OS Extended (Journaled). Disk Utility shows a compatible format by default.
Scheme: Choose GUID Partition Map.
Click Erase to begin erasing your disk and every container and volume within it. You might be asked to enter your Apple ID.
When done, quit Disk Utility.
5. If you want your Mac to be able to start up from the disk you erased, reinstall macOS on the disk.
REINSTALLING YOUR OPERATING SYSTEM
1 To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following sets of keys on your keyboard. Release the keys when you see an Apple logo, spinning globe, or other startup screen.
Reinstall the latest macOS that was installed on your Mac (recommended).
Upgrade to the latest macOS that is compatible with your Mac.
Reinstall the macOS that came with your Mac, or the closest version still available.
When you see the utilities window, you have started up from macOS Recovery.
2. When you're ready to reinstall macOS, choose Reinstall macOS from the Utilities window. Then click Continue and follow the onscreen instructions. You will be asked to choose a disk on which to install.
If the installer asks to unlock your disk, enter the password you use to log in to your Mac.
If the installer doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk first.
If the installer offers you the choice between installing on Macintosh HD or Macintosh HD - Data, choose Macintosh HD.
macOS Catalina installer window
Please allow installation to complete without putting your Mac to sleep or closing its lid. During installation, your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.
If your Mac restarts to a setup assistant, but you're selling, trading in, or giving away your Mac, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.
FOR MORE INFORMATION PLEASE SEE THE FOLLOWING
For more information on restoring or reinstalling your Mac OS please click here