my first suggestion is that you make a backup copy of your sources.list before editing it. i know it is broken now, but at least it is a starting point.
i would cd
into the /etc/apt directory:
cd /etc/apt
and then make that copy with cp
:
sudo cp sources.list sources.list.bak
.
i usually like to make sure the copy worked so i run the command ls
to see that sources.list is there as well as sources.list.bak
$ ls
apt.conf.d preferences.d sources.list.bak trusted.gpg.d
auth.conf.d sources.list sources.list.d
from there you will also be able to open the original for editing with gedit
the text editor:
sudo gedit sources.list
in order to make a line not be read (comment it out) you just need to put a # sign in front of it. i would suggest doing that for all of the lines that have disco in them. i don’t think you will need the eoan or trusty ones either, but one step at a time.
so a line like:
deb-src http://gb.archive.ubuntu.com/ubuntu/ disco-updates universe
becomes
#deb-src http://gb.archive.ubuntu.com/ubuntu/ disco-updates universe
obviously you need to save any changes you make to the file after you make them.
i would not suggest changing the whole thing, but for reference this is the sources.list file from a fairly new ubuntu 20.04 virtual machine where you can see that the only lines without a # in front of them have the word focal in them:
#deb cdrom:[Ubuntu 20.04 LTS _Focal Fossa_ - Release amd64 (20200423)]/ focal main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu/ focal main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ focal main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ focal-updates main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ focal-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ focal universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ focal universe
deb http://us.archive.ubuntu.com/ubuntu/ focal-updates universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ focal-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ focal multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ focal multiverse
deb http://us.archive.ubuntu.com/ubuntu/ focal-updates multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ focal-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu focal partner
# deb-src http://archive.canonical.com/ubuntu focal partner
deb http://security.ubuntu.com/ubuntu focal-security main restricted
# deb-src http://security.ubuntu.com/ubuntu focal-security main restricted
deb http://security.ubuntu.com/ubuntu focal-security universe
# deb-src http://security.ubuntu.com/ubuntu focal-security universe
deb http://security.ubuntu.com/ubuntu focal-security multiverse
# deb-src http://security.ubuntu.com/ubuntu focal-security multiverse
# This system was installed using small removable media
# (e.g. netinst, live or single CD). The matching "deb cdrom"
# entries were disabled at the end of the installation process.
# For information about how to configure apt package sources,
# see the sources.list(5) manual.
i don’t think you will have to reboot for that to work, but if you run
sudo apt update
and
sudo apt upgrade
and it still gives you a warning about the disco lines, you may want to reboot and try again 
as with all things system change related, i strongly recommend backing up your system before making any changes.