You have three options.
1. Uncheck the checkbox before clicking "update now" (see below)
2. Don't immediately upgrade, after 30 days you'll get this update silently (and without any third party software) if you have "Check for updates automatically (recommended)" set in your Flash Player control panel.
3. After receiving the notification update, close that dialog and download one of the stand alone installers from here: http://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-windows.html#mai n-pars_header
Hope this helps,