Just copy and paste this text in terminal

#!/bin/bash

mkdir ~/flashinstaller && cd ~/flashinstaller

apt-get purge flashplugin-nonfree flashplugin-installer gnash gnash-common mozilla-plugin-gnash swfdec-mozilla
rm -f /usr/lib/firefox/plugins/libflashplayer.so
rm -f /usr/lib/mozilla/plugins/libflashplayer.so
rm -f /usr/lib/mozilla/plugins/flashplugin-alternative.so
rm -f /usr/lib/mozilla/plugins/npwrapper*flash*so
rm -f ~/.mozilla/plugins/*flash*so

# download flash square and flash player from:
wget https://fpdownload.macromedia.com/get/flashplayer/current/install_flash_player_10_linux.tar.gz
wget https://download.macromedia.com/pub/labs/flashplatformruntimes/flashplayer11/flashplayer11_b1_install_lin_64_071311.tar.gz

tar xfvz flashplayer11_b1_install_lin_64_071311.tar.gz
chown root:root libflashplayer.so
chmod 0644 libflashplayer.so
cp -f libflashplayer.so /usr/lib/mozilla/plugins/
rm -rf libflashplayer.so
ln -s /usr/lib/mozilla/plugins/libflashplayer.so /usr/lib/firefox/plugins/
tar xvfz install_flash_player_10_linux.tar.gz
mkdir ~/.mozilla/plugins
mv -f libflashplayer.so ~/.mozilla/plugins/
cd