First decide which version of Debian you wish to upgrade to. The versions of Debian currently supported are as follows:

Version Code name Release date End of life date
 

9.0

 

Stretch June 17th 2017 approx. 2022 (LTS)
 

8.0

 

Jessie April 25th 2015 ~June 6th 2020 (LTS)
 

7.0

 

Wheezy May 4th 2013 May 2018 (LTS)

Information on upgrading Debian 6 Squeeze to Debian 7 Wheezy can be found on the web page accessed using the URL as follows:

https://www.howtoforge.com/how-to-upgrade-debian-squeeze-to-wheezy

Information on upgrading Debian 7 Wheezy to Debian 8 Jessie can be found on the web page accessed using the URL as follows:

https://www.howtoforge.com/tutorial/how-to-upgrade-debian-wheezy-to-jessie-stable-release

*** Note: Although it may be possible, it is not recommended to even attempt to do upgrades to a more than 1 version higher.