In the 'Luminaires' tab double tap on the luminaire that you want to change the default setting to. In the 'Luminaire settings' view, long press the ‘Default’ text under the ‘Modes’ and the ‘Edit’ option will appear. Set the light level and colour temperature for the luminaire to your liking. 

After changing these settings, you can simply tap anywhere else in the view to save the new 'Default' mode and return to the luminaire's properties. Next time you turn on your lamp it will automatically light up to the modified default light level and colour setting.