iOS: Tap the Settings icon and select Lights. From this page, you can re-order the lights and groups within a room by tapping Edit in the top right corner. To drag a light or a group, you need to press the horizontal bars icon on the right, the drop it in the space you want to move it to.

Android: open the menu and select Lights. From this page you can reorganize the groups and lights of the room by drag-and-drop.