Cannot upgrade Ubuntu 13.04 to 13.10 (W: Failed to fetch http://.../ubuntu/dists/natty-backports/...)

I cannot upgrade my ubuntu from 13.04 to 13.10

This is error while i try to upgrade:

W: GPG error: http://download.opensuse.org  Release: The following signatures were invalid: KEYEXPIRED 1386276565
W: Failed to fetch http://th.archive.ubuntu.com/ubuntu/dists/natty-backports/main/source/Sources  404  Not Found
W: Failed to fetch http://th.archive.ubuntu.com/ubuntu/dists/natty-backports/restricted/source/Sources  404  Not Found
W: Failed to fetch http://th.archive.ubuntu.com/ubuntu/dists/natty-backports/universe/source/Sources  404  Not Found
W: Failed to fetch http://th.archive.ubuntu.com/ubuntu/dists/natty-backports/multiverse/source/Sources  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

What have i tried already?

1. Unchecked something like ..from CD-ROM.. on Ubuntu Software Center > Software Source.


2. Tried to use this command "sudo rm -rf /var/lib/apt/lists/*"


3. Tried to change download server

How can i solve this problem?

At some point you added an apt source for that natty-backports repository you see mentioned in your error. This repository is no longer available for some reason.

Find this; it will be either a line in /etc/apt/sources.list or a file in /etc/apt/sources.list.d. Then delete the line or the file, save, and try again.