You can only delete folders that are empty. Before you begin, remove all the documents, dashboards, templates, or reports from the folder you would like to delete.
- Click Edit next to the folder name from any page that lists folders. On the Reports tab, click then Edit in the Folders pane. If you're not seeing the enhanced Reports tab, check that you're using a supported browser and ask your administrator to enable the new user interface theme.
- Click Delete or then Delete.
- Click OK to confirm.
For additional Information see Force.com's: Deleting Folders