The fact that there does not seem to be a way to change the default startup project in a Visual Studio solution has been bugging me for a while. Fortunately it appears that I have finally stumbled on the/a solution…
If you edit the sln file in a text editor, you will find a bunch of Project/EndProject statements towards the top of the file. It appears that the default startup project is the second project listed in this section (the first project, in my case at least, is the “Solution Items” project). This means you can manually move the appropriate lines for the project up to the second position and thus change the startup project.
Quite why there is no way of doing this in the UI is beyond me…