To do so, go to Marketing-> General Settings tab-> Header card. There are two ways to Add a Menu Item. By clicking on Add a Menu Item, you can select Add Page or Add Custom URL. Via Add Page, you can add the pages that are already created in the system easily. By Add Custom URL, you can add either Zeevou Pages (that can be added via Add Page easier), or other pages.
Note that if any of them have already been there, you can edit them by clicking on the pencil.