en.txt
The menu editor changes what the menus contain: move entries up or down, remove them, or add new ones.
Find it in Settings under Customise, as Customise menus. Choose a menu, then an entry to move or remove it; Add an entry puts in a new one. Changes apply at once. Reset this menu returns a menu to how its plugins define it, Reset all menus does so for every one.
You cannot lock yourself out: an entry that is the way to the settings stays until you add another way to them.