I need to stop an iMac update that is already downloaded and ready to install I am testing software on an El Capitan system. Unfortunately it had the download in the background option checked and now it has Sierra "ready to install".
The options are now, tonight or tomorrow.
I need to stop this update entirely.
I've searched my entire system for the installer file but cannot locate it. I searched for any file downloaded in the last day and sorted by size. There's nothing there that looks like a Sierra OS installer.
I turned off all the auto update options in System Preferences. I wonder if that will cancel the update from proceeding? I'll know tomorrow when I find out if it asks me again or not.
But if you know how to stop it immediately please explain how to do it. Thank you.
The installer is usually in Applications. "Install macOS Sierra.app". Delete that, empty the trash (or direct-delete in Terminal).


  I am testing software on an El Capitan system


Get a virtual machine like Fusion or Parallels. Set up a new install of the OS and make a snapshot. Whatever happens next can be restored to the initial state in 30 seconds by restoring the snapshot.

VMs are also capable of running the next version. I run El Capitan right now and have VMs from 10.5 to Sierra. And with enough memory it's possible to have them all open at once.