If your Ubuntu is too old and aptitude stopped working, showing 404 not found.
Add this to /etc/apt/sources.list, changing the CODENAME to your Ubuntu version.
## EOL upgrade sources.list
# Required
deb http://old-releases.ubuntu.com/ubuntu/ CODENAME main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ CODENAME-updates main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ CODENAME-security main restricted universe multiverse
# Optional
#deb http://old-releases.ubuntu.com/ubuntu/ CODENAME-backports main restricted universe multiverse
That’s enough.
Read more if you’re interested in.
https://help.ubuntu.com/community/EOLUpgrades
http://old-releases.ubuntu.com/releases/