An Introduction to Exporting Products
You can use the Import/Export > Export Products section of the control panel to export your product list in the Microsoft Excel Spreadsheet format. You could choose to export your products into an Excel file, edit the data manually, convert it into a CSV file, and then import the products back into your store via the Import/Export > Import Products section. Exporting and Importing products in this way would allow you to perform a data merge (i.e. updating your existing products via an import, rather than adding them as brand new products).