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.
Alternatively, you can simultaneously set the default light levels for all luminaires in your network by setting the luminaires to their desired dimmed levels/colour temperatures (either individually or by using the ‘All luminaires’ button in the ‘Luminaires’ tab. Then go to the ‘More’ tab and select the ‘Configure all luminaires’ option under ‘Network Setup’. Then select ‘Save current state’ and then ‘Startup -> Default mode’.