Using Spreadsheet_Excel_Writer to update .xls file

Hi Pear Developers, I recently had Spreadsheet_Excel_Writer installed on my system because I believed I could use it to update an existing .xls file. Now that I look at it more closely, I only see examples of how to create and save a new spreadsheet. Is there a way to use Spreadsheet_Excel_Writer for updating? If not, is there another package that can do this? Thanks, Lisa