Upgrading from Debian 7 to 8 is reasonably straight forward, following the official instructions. These shorter summaries are also useful references.

Very briefly then, make sure you have backup.
dpkg --get-selections "*" > dpkg_selections.txt
tar zcvf upgrade_backup.tar.gz /etc /var/lib/dpkg /var/lib/apt/extended_states /etc/mysql/my.cnf /etc/fuse.conf /etc/ssh/ssh_config

Update /etc/apt/sources.list, and replace all occurrences of wheezy with jessie.
sed -i 's/wheezy/jessie/g' /etc/apt/sources.list

If VirtualBox is installed, update to the new key:
wget -q -O - http://download.virtualbox.org/virtualbox/debian/oracle_vbox_2016.asc | sudo apt-key add -

Then comes the upgrade dance, with a few prompts, warnings, questions.

apt-get update
apt-get upgrade
apt-get dist-upgrade

After the upgrade, it is recommended to purge unused packages
apt-get purge $(dpkg -l | awk '/^rc/ { print $2 }')
apt-get autoremove

It is also recommended to install the linux-image-* metapackage, e.g. for AMD CPUs
apt-get install linux-image-amd64

Finally, cross your fingers and reboot.