When I try to update using update manager I get :
Requires installation of untrusted packages The action would require the installation of packages from not authenticated sources.
When I try to install any thing via software center I get the same error but this time with an option of
Failed to download repository information Check your Internet connection.
I looked for some solutions for this problem and one of them was to change the mirrors (servers) but when I try to change by choosing a specific server nothing happens ,the problem still there.
And when I choose:
select best server I get :
No suitable download server was found Please check your Internet connection.