How to use Thunderbird with GUMail: Editing Filters
Start Thunderbird, click on Tools (at the top of the screen), then click on Message Filters...
This will bring up the Message Filters window.
Click on the filter you want to modify, then click on the Edit button.
This will bring up the Filter Rules window.
Make the changes you want (in this example, we're changing the filter so it deletes all mail that's sent from andersow and has aardvark in the subject). Then click on the OK button to save your changes.