You can access it two ways. First, you can edge-swipe from the left side of the screen to bring up the Radar NavData box. There you will see where you can switch from auto to manual with a tap to adjust your Gain, Sea, and Rain.
Alternatively, you can edge-swipe up from the bottom and on the first tab [Radar] you will see the Manual Gain, Manual Sea, and Manual Rain. Tap on the setting you want to adjust, then use the slider.