After the modification of /etc/apt/sources.list that you made, the (graphical
or textual) update to Trisquel 8 described on
https://trisquel.info/en/wiki/keeping-system-updated still does not work?
If so, I would simply try to substitute "belenos" with "flidas" in
/etc/apt/sources.list:
$ sudo sed -i 's/belenos/flidas/' /etc/apt/sources.list
Then update:
$ sudo apt update
And finally upgrade:
$ sudo apt upgrade
For your piece of mind (mainly), it is worth doing a backup of all user data
before all that.
