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.