Go to your Spotlight search bar and type in “Terminal”

Once that opens copy the following text and paste it into the terminal and press enter:

rm -rf ~/Library/Preferences/Microsoft/Office\ 2008/Microsoft\ Office\ 2008\ Settings.plist ; rm-rf /Applications/Microsoft\ Office\ 2008/Office/OfficePID.plist

This will remove the serial number and you will be prompted for a new serial number when you open any Office product.