To manage the folders for one or more references:

Select the references and select Assign. You can assign the selected references to an existing or new folder or remove them from a folder.  You can also copy selected references to another Project.

Folders appear in the My Folders tab in the Navigation area. The number of references in a folder appears after each folder name.

When the My Folders tab is expanded, you can drag and drop references to a folder. In addition, you can use this area to add a folder or a subfolder, or rename, delete, or share a folder. You may also Move or Copy the folder to another Project or Find Duplicates in the folder. There is no limit for number of folders, but subfolders can be up to three levels deep, for example: main > sub1 > sub2 > sub3. 

Full Instructions:  https://knowledge.exlibrisgroup.com/RefWorks/Product_Documentation/RefWorks_User_Guide/0400_Managing_References#Organizing_References_by_Folders