Reset VO2 max

You can't reset it. Just continue to run and it will eventually get to the correct value. Check the heart rate values and HR zones so they haven't changed. You will probably have to do some faster runs to get the VO2 max to move up faster.

Don't let anyone use your watch. It is personal and will adapt to what it reads from the sensors. Get your wife a watch as a gift.

Sorry to trouble you but don’t know how to start a thread. I have just started using a new Garmin Vivosport as the old one got water damaged. My fitness age has gone from 20 to 57 and VO2 Max from 41 to 31, even though my run today was faster and heart rate lower. Do I need to test it first or just keep running until it is more accurate? 

Hi, a new device needs to "learn" your fitness level by collecting data from your activities. To ensure better VO2max estimates, include some high-intensity workouts in your schedule (high heart rates). Also make sure the max HR set in your device reflects your actual max HR (do some max HR workouts / tests if necessary -- don't rely on "220 minus age" or any other random rules of thumb, they can be very inaccurate). Cheers

Thank you. 

Sorry to trouble you but don’t know how to start a thread.

BTW, for that problem, see the button +New at the top right of the forum you want to post to. 

Otherwise, the safest way to get an accurate VO₂max value is by conducting the Guided Lactate Threshold Test with a HRM chest strap (appart of a lab test). Using just the wrist optical HRM built in the watch is always little bit a lottery.