Make sure that you are logged in to your site and then navigate to the content that is the menu you want to delete. The easiest way to do that is to just click on the item in the menu. Once you are on the correct page, click the 'Edit' tab at the top of the page.
Scroll down towards the bottom of the page and click 'Menu settings' to expand it.
Note that because this page is already set to be a menu the information is already filled in. And there is also a check box at the bottom of the settings box to delete this menu item. Just check the box, scroll down and hit submit. You will not receive a confirmation question asking if you're sure - the menu is deleted immediately upon hitting submit. So please make sure you're sure.
TIP: If you delete a top level menu the menus below it will also disappear! They no longer have their parent so they don't know where they're supposed to be. So if you want to switch menus, create the new one first, change the 'Parent item' for the other existing menus and then delete the old one.
If you accidentally delete a parent menu before creating (and assigning) a new parent and can't find the lower menus, just use the content list to find the lost pages so you can edit them back into a menu.
