As "root" or using sudo (place the command 'sudo' before all commands below if not already root...
What flash plugin packages are already installed?
apt-cache pkgnames|grep -i flash
You might see "flashplugin-installer" in the list. flashplugin-installer installs the 32-bit version of Flash and a related compatibility library which may be slower and less stable, so it’s recommended to install the adobe-flashplugin package from canonial partners repository.
To enable the partners repositor open the sources.list file
vi /etc/apt/sources.list
Add the partner repositories by removing the # in front of the following lines
deb http://archive.canonical.com/ubuntu precise partner
In this example we are using Ubuntu Precise (Precise Pangolin). If you're using another version simply replace "precise" with the correct string.