When hovering your mouse over a stack, you will see a Toggle Editing of Stack Elements icon:
By clicking on this icon, the Drag icon will appear within all elements that make up the stack. By grabbing the Drag icon with your mouse, you will be able to move that element to another location within the stack. The Site Editor will give you a target of where you can move the element to. It is highly recommended that you only move the it to where the Site Editor suggests.