Hi Anandawardhana, I had exactly the same issue as you.
My advice would be to download Trisquel 7.0 from trisquel's official website (https://trisquel.info/en/download) or through mirror website maintained by FSF. (http://mirror.fsf.org/trisquel-images/)
You can also download Trisquel 8.0 Development Image from former link, if you want to test the latest software.
It is not possible to upgrade from 6 to 7 now, as Trisquel 6.0.1 is no more actively maintained. (No more package update (in particular security fix),and bug reports are now ignored.)
