Visit the pages below for information about upgrading to later versions of Unity.
Upgrading to Unity 5.3 [Released 08dec15]
Upgrading to Unity 5.2 [Released 08sep15]
Upgrading to Unity 5.0 [Released 03mar15] (There are several sub-pages to this.)
Upgrading to Unity 4.0 [Released 13nov12]
Upgrading to Unity 3.5 [Released 14feb12]