$ sudo apt-get update
$ sudo apt-get install flashplugin-nonfree 

flashplugin-nonfree is installed in your system.

Make ensure the flashplugin-nonfree package were installed using the commands given below,

$ sudo dpkg-query -l | grep flashplugin-nonfree *

You will get with flashplugin-nonfree package name, version, architecture and description in a table

flashplugin-nonfree package basic information: