In a terminal window you type (or copy & paste) the following command line
sudo apt-get install ubuntu-restricted-extras
Then there is a prompt to enter your password (the same one as you use to log in to your user account). This assumes that you have superuser privileges. Otherwise you are not allowed to install programs like this. I get the following prompt, and you get a prompt for your user ID.
[sudo] password for sudodus:
There is no echo (no stars, nothing) when you enter your password, but it is read anyway, so
- enter your password
- and finish with the Enter key.
Then things start to happen. (I have an up to date version of lubuntu-restricted-extras, so I get the following output.)
Reading package lists... Done
Building dependency tree
Reading state information... Done
lubuntu-restricted-extras is already the newest version (65).
Edit: After writing the answer, I saw your latest comment. I suggest that you try to repair your system according to the following link (the tips using command lines)
askubuntu.com/questions/885453/16-04-software-download-stuck-pending 'From the command line'
adobe-flashplugin
. With that you get flash for both Firefox and Chromium/Opera/Vivaldi. – Gunnar Hjalmarsson Mar 02 '17 at 08:42