apt-get update not working (Not Found [IP: 91.189.92.201 80]) [closed]
Solution 1:
Natty no longer supported or gets updates. You need to change from the archive.ubuntu.com to the old-releases.ubuntu.com server, which is the intention of the sed command you mentioned (I have not had chance to confirm if that works or not). Open and replace all entries in the following file:
sudo gedit /etc/apt/sources.list
Then after run the update and upgrade commands as you would normally.