It is simple, but i am issuing no guarentees, this will probably work for you, but there is a chance that something could go wrong, you have been warned
Loosing network connectivity: One thing that sometimes happens is that the Ethernet number can change from eth0 to eth1 for example (this could happen if your system has not been exposed to other cards, in which case it can turn from eth0 to eth6), What you can do about this is either be physically present to fix it (fix in /etc/network/interfaces) or add another interface to the file so that it can work out of the box, or have KVM over IP, or something of the sort, if you have LXC installed, the containers will not be able to fire up because they must have the following two extra lines in their config, /var/lib/lxc/CONTAINER_NAME/config
lxc.autodev = 1
lxc.kmsg = 0
apt-get update
apt-get dist-upgrade
edit your apt sources (vi /etc/apt/sources.list)
replace the word wheezy with jessie (Wherever you find it)
apt-get update
apt-get dist-upgrade
You should be done