Before you dist-upgrade your system, use Ctrl-Alt-F1 to log in to the tty console as root. Then do these steps:
init 3 (shuts down X)
apt-get update (observe that it fetches package lists -- sometimes if a repo is in the process of being refreshed, it will "fail to fetch". Just wait 5 minutes and try it again.)
apt-get dist-upgrade (observe it wget the upgraded packages, unpack and install them, and reconfigure the initrd image and or update grub for new kernels)
apt-get clean (deletes the download files in the apt archive)
init 5 (brings X back up and your login manager)
When you are in X, you can open your root terminal (Alt-F2 "kdesu konsole" for KDE), and in that terminal you can apt-get update and then apt-get dist-upgrade -d to download the upgrade but not install it. Then later you can get out of X and run it on the console and install it.
New siduction users who are accustomed to doing updates in X in other distros sometimes find it dismaying that they need to exit to the console, but it is the nature of sid that new packages related to the xserver and installed video driver might break your system if X is running at the time they are being installed and configured.