Selecting which project under a solution to debug or run in Visual Studio 2010

You can right-click a project and set it as the Startup Project. It then is debugged when you press F5, or run when you press Ctrl-F5.

You can right-click a project and select Debug / Start new instance to debug a project that is not the startup project.

On a side note, you can run / debug the unit tests without setting the unit test project as the startup one. There are icons (and keyboard shortcuts) in the toolbar to run or debug unit tests at various scopes (if not visible right-click your toolbar and select Test Tools).

You can right click on the project and choose to set as startup project