UKanDo now uses the Linden Lab® Viewer Version Manager (VVM) system for automatic updates and version management. As a user you have the option to set automatic or manual updates in Preferences-> Setup-> Software updates (by default it is set to Install automatically).
The release candidates tick-box is disabled as there are no plans what-so-ever to use this feature.
VVM is also used to control which versions can or can’t login to Second Life®. The versions which can login are the Current Release plus the previous release, all versions before these are now blocked!
One side effect to this is as a new version is released the older previous release will then become a blocked version.
As a user, this is not something you need to worry about! If you are inadvertently using a blocked version, it should automatically update to the latest release before it allows you to login.