What are my next steps if my computer hangs up during the new OS software download process? A pop up appeared for me to close Safari, then it stopped and I cannot reboot with it mid-stream.
It's not common for there to be a hang related to Safari being active; that's more typical of a (failed) patch installation process, and something having gotten tangled.
To attempt to clear Safari, press and hold the option key ⌥ and click and hold (option-click) on the Safari icon in the dock, and (when the pop-up appears) select Force Quit. Alternatively, Command-Option-Escape ⌘⌥␛and select Safari from the pop-up, and kill it.
If that fails (and after you've closed everything else) and you can't get Safari to exit, then press and hold the power button for about 10 seconds to force a shutdown.