How to "un-maximize" or "restore" an Excel window
The Escape key un-maximizes Excel. Another way is to push the mouse cursor to the top edge of the screen. The menu bar would appear. A blue unmaximize button will appear in the top right corner. Finally, you can use Control Command F. That should work with all programs, not just Excel.

If you have an additional monitor and set up access to it in the "top" direction, it will not be possible to "hit" the top of the screen to get the menu bar. In this case hit the top corner of the monitor.