There are two ways to upload files to a folder using the Web App.  First, you can right-click on any folder and select "Upload."  Second, you can select a folder and click "Upload" in the menu bar at the top of the screen.

Either method results in an upload window where users can add files to be uploaded to that folder.  To add files, either drag them from your local file system to the window, or click "Add files" and select the files to add.  Once you have added all of the files you would like to upload, select "Start Upload" and the files will be securely transferred to your folder.