In the Links panel (hint: "Drag to reorder") grab any row and drag it up or down to change its order, or drop it slightly to the right under another row to nest it as a child - that parent becomes a dropdown on the site. Nesting works to several levels (the site renders top-level items, their children and one further sub-level). Every drop is auto-saved; you can also press Save menu order at the bottom to save explicitly and reload. The count pill next to "Links" shows how many items the group has.