This is on purpose. The latest internal builds are not fully tested and are not ready for production. They are meant to give an idea to the developer whether a bug has been already fixed.
You should manually update your project to point to the new assemblies.
Still, if you need to use the LIB in all subsequent projects - you can extract the files (and override) over the files of the current installation (in C:\Program files ... )
So, next time you add a reference to that location - the newest build will be used.
the Telerik team
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to help you get started with RadControls for Silverlight