Upgrade to Debian Jessie

Only do this if you know what you are doing.

Backup your old sources.list:

cp /etc/apt/sources.list{,.backup}

Replace all occurrences of wheezy by jessie, update the package list and run the upgrade:

sed -i "s/wheezy/jessie/g" /etc/apt/sources.list
aptitude update
aptitude dist-upgrade"