Select the paned window.
Select a paned window by clicking at its edge. Be sure you select the paned window and not one of its panes. You will know you have selected the paned window if a dark box is drawn around the paned window.
Or, open the Module Browser and select the paned window there. This is the easiest, surest way to select a paned window.
Choose Unmake Paned Window from the Layout menu or from the pop-up menu in the interface or the Module Browser (displayed by pressing mouse button 3).
The panes that made up the paned window will become separate panes again.