It can not be upgraded

Need some help please!
When i tried to upgrade the system i received following message:
Could not download packages
404 Not Found
404 Not Found
404 Not Found

Can you help me please,what to do?
Thank you!

This topic has been discussed a few times already. You changed the sources.list with some kind of tool that did it wrong. Please change it back to the original state.
See this thread for example:

Ok thank you