What commands did you actually try, and what was the full error message (=> what was the file that "was not found"?)
This should work if everything is fine:
sudo apt-get update
sudo apt-get upgrade
Or opening the graphical app and clicking the update button. If you get errors, you should post them here, or at least enough of them to tell what's wrong. If the error message says that it can't download/find some reposity files, it may be that the mirror you are trying to connect to is down or unaccessible. In that case you can simply try another mirror; check out the reposity list in Adept (or the plaintext file) and see if there are other mirrors commented out; if yes, try uncommenting some, and commenting out the ones you were using until now. Or head to Ubuntu/Kubuntu website and see what the mirror addresses are. But if the "file not found" error is about something else than mirrors being unaccessible, don't go changing the reposity config files.