Make Debian Upgrade

Remount SD-Card:
mount -o rw,remount /dev/root

Configure NIC to acquire configuration via DHCP:
Add lines to /etc/network/interfaces:
auto eth0
iface eth0 inet dhcp
And remove every other lines regarding eth0.

Set up NIC:
ifdown eth0
ifup eth0
NIC should acquire a new DHCP Lease now.

Configure sources for Upgrade:
Add lines to /etc/apt/sources.list:

deb http://ftp.de.debian.org/debian/ etch main non-free contrib
deb-src http://ftp.de.debian.org/debian/ etch main non-free contrib
deb http://security.debian.org/ etch/updates main contrib non-free
deb-src http://security.debian.org/ etch/updates main contrib non-free


Repair package list:
cp /var/lib/dpkg/status-old /var/lib/dpkg/status

Get new package list:
apt-get update

Do upgrade:
apt-get upgrade