Georgie,

The key piece of data you neglected to mention, is how many details are are in this active project, as that will impact the best solution. Overall, the first thing you'll want to do is to run a Project Report from the Projects dialog, which will list out what detail folder is assigned to each project. You can also run a Project Organization report from the Organization tab, which will help in planning out how best to organize the detail folders going forward, and which project folders need to be moved.
As for correcting the issue at hand, assuming the number of details in the active project is under 50, the recommended method would be to individually drag-and-drop each detail from where they are to where they should be. Then once they are all moved, you can reassign the project detail folder, and the system will update automatically with the revised location.
The Detail Explorer does also support dragging and dropping entire folders at a time, but that comes with a giant warning, as there are so many fail points for an operation that could be affecting hundreds of details and multiple projects.

--J

Georgie, here's some steps to untangle all this.

1. Create a new folder for the issue project, in the correct place.
   
2. Use the detail explorer to drag/drop MOVE that project's details from the wrong top level folder to the correct new folder. Moving will keep the associations to the referenced project refnotes, plants and callouts.
   
3. In project list, edit the issue project, and under the detail folder browse and select the new folder location. This is how you change the assigned detail folder of a project.
   
4. Then you can continue to assign other projects to their correct detail project folders.
   

-Amanda