To set the printer that GreenPrint will print to by default, go to Start > Control Panel > Printers & Faxes (or just Printers in Vista), right click on the GreenPrint printer and go to Printing Preferences, then under Target Printer choose the printer that you print to the most often with GreenPrint. Hit Apply and OK. Please then restart any applications (Internet Explorer, Word, etc.) that you had open so that the default target printer setting takes hold and shows up as an option in GreenPrint.
If you'd like to change the output printer for an individual print job, but don't want to change your default setting, you should do this by going to File > Print within the application you're using (Word, Firefox, etc.). On the print dialog box make sure GreenPrint is selected as the printer, then click on the Properties/Preferences button, and select the target printer there. This setting will only apply for that session within the specific application you're using.
With the NEWEST releases of World and Home Premium, you can also change your 'Target' printer from within the GreenPrint preview window. To do this, start a print job. Once the GreenPrint preview is shown, pull down on the 'Help' menu option and choose 'Welcome Screen'. Select your desired, most-often-used printer. You can also watch a ~5 minute tutorial video from this screen.
*GreenPrint World is discontinued.