One of the choices on the Menu Bar that runs across the top of the screen is WINDOW.  When you click on WINDOW, a list of available windows appears on the lower part of the menu list.  The current window may be the only one on the list, but as you open other parts of the program, other windows will be included on the list.

You can move back and forth between different windows by clicking on the appropriate window from the list.  For instance, you may work in the Vendor Database.  Occasionally, you may want to re-look at a couple of purchase orders on the Purchase Order Screen.  To go to the Purchase Order Screen, select WINDOW >> MAIN PURCHASE ORDER SCREEN.  Once you are through looking at the purchase orders, you can return to the vendors by selecting WINDOW >> VENDOR DATABASE.