Select a file.
Click 
and select Upload a New Revision from the pop-up menu.
In the dialog, drag the files from an Explorer window to the central area or click in the central area to display the Open dialog to browse and select the files.
The file being uploaded does not have to have the same filename as the file you are uploading to. When the names are different, a confirmation is required and then the contents of the old file are replaced with the contents in the new file, but the file name remains unchanged.
Because you are making a change to the contents, an electronic signature is required.