Run the following commands in a terminal to remove existing Flash files:

sudo apt-get remove -y –purge flashplugin-nonfree gnash gnash-common mozilla-plugin-gnash swfdec-mozilla libflashsupport nspluginwrapper
sudo rm -f /usr/lib/mozilla/plugins/*flash*
sudo rm -f ~/.mozilla/plugins/*flash*
sudo rm -f /usr/lib/firefox/plugins/*flash*
sudo rm -f /usr/lib/firefox-addons/plugins/*flash*
sudo rm -rfd /usr/lib/nspluginwrapper
sudo rm -rf /usr/lib/flashplugin*

Then run the following command to install the correct version of Flash:

sudo apt-get install flashplugin-installer