To edit the settings in Custom Mode, tap the Modes button on the bottom of the main screen of the app with the temperature dial:
Select Custom Mode for cooling or warming and tap Edit:
Select your wave sensation and session duration, then tap Apply to save your settings:
Tap the X to return to the main screen of the app. A single tap on the cooling or warming side of the light bar will start a new session at your last used temperature level, or you can hold down the - / + side to adjust the temperature.
Your Custom Mode settings will be saved for you, and you can adjust them any time by tapping the Edit button.