You can toggle the use of menu bars with M-x menu-bar-mode.
With no argument, this command toggles Menu Bar mode, a global minor
mode. With an argument, the command turns Menu Bar mode on if the
argument is positive, off if the argument is not positive. To control
the use of menu bars at startup, customize the variable
Expert users often turn off the menu bar, especially on text
terminals, where this makes one additional line available for text.
If the menu bar is off, you can still pop up a menu of its contents
with C-mouse-3 on a display which supports pop-up menus.
Or you can enable
context-menu-mode and customize the variable
context-menu-functions to pop up a context menu with
mouse-3. See Mouse Clicks for Menus.