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 run the billing portion of the program and create billing statements.  After creating the billing statements, you may want to re-look at a couple of invoices on the Invoice Screen.  To go to the Invoice Screen, select WINDOW and "THE MAIN INVOICE SCREEN."  Once you are through looking at the invoices, you can return to the statements by selecting WINDOW and "BILLING STATEMENTS."