In Update Manager I try to update the list of packages and I get a window titled synaptic with the following output
Failed to fetch http://security.ubuntu.com/ubuntu/dists/raring-security/main/binary-amd64/Packages 404 Not Found [IP: 2001:67c:1360:8c01::18 80]
(many more lines like this)
Some index files failed to download. They have been ignored, or old ones used instead.
How do I fix this? Please answer with complete step by step instructions. Thanks.