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

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