1) Command-Option-Esc will bring up the Force Quit window. Select the installer and quit. 2) Open Activity Monitor in Applications/Utilities. In the top portion of the Activity Monitor window, find the installer then click on the red icon to quit process.

How do I force quit a Mac Installer?

Press these three keys together: Option, Command, and Esc (Escape). Or choose Force Quit from the Apple menu  in the upper-left corner of your screen. (This is similar to pressing Control-Alt-Delete on a PC.) Then select the app in the Force Quit window and click Force Quit.

How do I cancel a Mac installation?

Canceling the download is possible by holding down the option key. The “Pause” buttons change to “Cancel” buttons.