Unfortunately, you cannot change the Menu Type after the Menu has been created.
Fixed Price and A La Carte Menus are fundamentally very different in terms of setup. It is therefore not possible to convert the information from one layout to another.
If you do have a Menu with lots of Items which you need to change from one Type to another, always create the new Menu first before you delete the current Menu.
This is because when adding Items to the new Menu, these Items already exist on the system. You can assign the Item to the new Menu by typing the first few characters of the Item Name in the Item Name field, and clicking on the Item when it appears below.