updating from such a repository can't be done securely, and is therefore disabled by default.

Question

Problem :

I am facing following ubantu issue

updating from such a repository can't be done securely, and is therefore disabled by default.

Answer 1

You should try to set options in your sources.list which should be located at /etc/apt/sources.list as follows:

deb [trusted=yes] http://www.deb-multimedia.org jessie main

This trusted option is responsible for turning off the GPG check.

Now you can either edit the file within the terminal with vim  or any non-terminal editor like gedit.