Upgrade
Upgrade debian
STEP 1 CHECK YOUR CURRENT DISTRIBUTION
nano /etc/debian-version
current version (sept 2015) 7.8
table of reference:: table with colors https://en.wikipedia.org/wiki/Debian%7Chttps://en.wikipedia.org/wiki/Debian
check :: wheezy old stable
STEP 2 UPDATE CURRENT DISTRIBUTION
old stable > to stable
apt-get update && apt-get dist-upgrade && dist-upgrade && apt-get autoremove
(first update than upgrade, than upgrade again if something didn't happen well and autoremove the packages which are not used)
STEP 3 UPDATE THE SOURCE.LIST FOR THE NEW DEBIAN RELEASE
update the source.list
nano /etc/apt/sources.list
wikipedia / debian > table with colors (change the name of the distribution)
https://wiki.debian.org/SourcesList%7Chttps://wiki.debian.org/SourcesList
deb http://www.debian.org/debian jessie main
deb-src http://www.debian.org/debian jessie main
STEP 4 UPDATE THE SOURCE.LIST FOR THE NEW DEBIAN RELEASE
apt-get update && apt-get dist-upgrade && dist-upgrade && apt-get autoremove