The resting heart rate is being updated all the time. However, my zones are % of HR Max, which is just age - 220. Zones didn't seem to change based on updated max heart rate possibly being found during exercises. You can change your HR zones to % HR Reserve, which I tried, and that asked for a manual entry of your resting heart rate, meaning it probably doesn't update automatically. I decided to just leave my zones as %HR max, though.