How can I choose the default workspace?

I'm a developer working out how to use the APIs in a sandbox instance. Recently I discovered (be gentle, please) that when an instance contains multiple workspaces, API calls need to indicate which workspace they're intended to operate on, and when they don't include such an indication, they operate on the "default" workspace. I'm going to change my code to make the workspace of choice explicit, but it will take a while for me to roll that change out. In the meantime, as a stopgap, I'd like to change which of the workspaces in this instance is the default for the instance. How do I do that?