To delete the file / folder:

Go to your project and from your project-specific reports, open the Files/Documents report.
Point to the file/folder and click on the actions drop-down and navigate to More and then click Delete.
Confirm the deletion by clicking on Yes.

To move the file / folder:

Go to your project.
On the left hand side of the window, click files from the list of project specific reports panel.
Point to the file/folder and click on the actions drop-down and navigate to More and then click Move.
Select the Project and Folder where you want to move the file.
Click Next.

You can delete, move the files on tasks the same way as you do it for the projects.

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