Change macOS System Version

Follow this procedure when you need to update macOS from a version prior to 10.8 to a more recent OS.

Make sure a password is set up for the admin user. THIS WILL NOT WORK IF THERE IS NO PASSWORD.

1. Log in to the customers admin user account.
2. Open Terminal.
3. Copy and paste the following command: sudo nano -w /System/Library/CoreServices/SystemVersion.plist
4. Press enter
5. Enter the admin password (it will not show any characters as you type!) and press enter.
6. Use the arrow keys to navigate to the end of the version number where shown highlighted below. Replace this number with 10.8.6
   
7. Use the arrow keys to navigate to the end of the second number shown below and replace again with 10.8.6
8. 
   
   
9. Press control + x to exit and when prompted to save modified buffer type y to save.
10. Press enter to accept the default file name.
11. Exit Terminal.
12. Restart and run the OS installer that you wish to upgrade to.