You may try the offline installer. Follow these steps
1)Download the uninstaller from: Uninstall Flash Player for Mac OS
2) Open the page: Installation problems | Flash Player | Mac
Under the heading "Still having problems" click on the specific link to NPAPI/PPAPI plugin.
3) Ensure all the browsers are closed. Run the uninstaller and then the offline installer.
If the problem persists, please write back to us.