To open a menu:
Select Open Menu from the File menu; the Open Menu dialog appears.

Fill in the dialog, specifying the database, mapset, language and menu number.
Although menus are really just maps, initially the menu combo box will only display maps that have been created as menus. If you check the "All Maps" checkbox the combo box will display all the maps in the mapset. This allows existing maps to be converted to menus.

Edit the menu to the desired state; for details see Creating a New Menu.