When downloading a .dmg from Apple's webpage illustrated above, what is the best practice, or order of operation, in the following scenario:
I have an iMac with 10.10.5 Yosemite, that I want to update to 10.11.6 El Capitan (yes, in 2023).
But, and this is a big but, I do not have administrator privileges to the iMac.
However, I DO have the original discs that shipped with the iMac.
But, and this is an even bigger but, I am afraid that the original discs will take the iMac back to a jungle cat.
And I am worried that an ancient enough jungle cat may never make it back to a California National Park.
I am currently using the iMac as a guest until I figure out how to manage wiping the drive and loading the latest MacOS that will run reasonably on a 12 year old machine.
I have access to external drives, if that would help in terms of loading a .dmg file to.
I want to entirely delete the personal data of the prior owner who gave me the machine, as I promised to.
What is the best way to proceed?