Check out the Extending Report Designer help article that elaborates on the topic.
Generally if you directly reference the assembly from Visual Studio project's bin, restarting the standalone report designer will reload your updated assembly in the application domain.
the Telerik team
See what's new in Telerik Reporting Q1 2013. Register for the March 4 webinar to witness the impressive new visualizations in Telerik Reporting. Just for fun, 10 webinar attendees will be randomly selected to win a Telerik T-shirt and a $50 Gift Certificate to ThinkGeek. Register now! Seats are limited!