If (like most of us), you are not familiar or comfortable with SQL queries to do mass changes to your data, then (particularly for product-related data), you could use Easy Populate (version 1.2.5.4 is appropriate for ZC 1.3.8a).

This program enables you to export your product data to a Tab-Delimited TXT file, which can then be opened and edited in a spreadsheet program, such as MS Excel, or (better) OpenOffice.

Making quick and otherwise time-consuming changes is much easier in a spreadsheet, where you can use the spreadsheet's many functions to duplicate cell data, perform calculations and re-organise records. When you've edited the data in the spreadsheet, it is then saved again as a tab-delimited TXT file and through the Easy Populate interface in your admin, is then used to "update" the affected product data.

I have written a WIKI on Easy Populate to help novices and you can find it here.

Easy Populate is excellent for product data management purposes, as long as you appreciate its limitations and follow its rules. The WIKI item takes you through the most important of these.