Visit the pages below for information about upgrading to later versions of Unity.
Upgrading to Unity 5.5 [Released 30 November 2016]
Upgrading to Unity 5.4 [Released 28 July 2016]
Upgrading to Unity 5.3 [Released 08 December 2015]
Upgrading to Unity 5.2 [Released 08 September 2015]
Upgrading to Unity 5.0 [Released 03 March 2015] (There are several sub-pages to this)
Upgrading to Unity 4.0 [Released 13 November 2012]
Upgrading to Unity 3.5 [Released 14 February 2012]