VO2 Max and how to improve it

Yeah mine is measured but I just feel it’s way off. It has me down as a 60 year old which doesn’t seem right. 
I will try a run. Does a treadmill run work or does it need to be outside with GPS ?

Thanks. 

I wonder if, in addition to looking at your heart rate at the end of a period of exertion (a key factor in many VO2 formulas), Garmin's calculations for estimated VO2 are also taking into account your resting heart rate using this formula: 
VO2 max = 15.3 x (MHR/RHR) 
MHR = Maximum heart rate (beats/minute) calculated using age = 208 - (0.7 x age)
RHR = Resting heart rate (beats/minute)

If that's the case, and if your resting heart rate is high for any reason – regardless of your actual age and self-perceived fitness – then that could lower your VO2 and correspondingly raise your fitness age considerably. 

Is there any way of updating one's V02 max manually? What if I had a measured V02 max that is more accurate than the calculated one from Garmin?

The Vivoactive 4 will definitely report an estimated VO2 max for you based on fast walking (this assumes you begin a Walk activity on your watch) so I would assume that it takes cycling and walking activities into account in addition to running. 

In order to improve your estimated VO2 max in the short term (i.e. weight and age remaining unchanged) the two variables available to you are: 
a) reduce the time it takes you to run or walk a given distance, and/or 
b) become more efficient (cardio/technique) so that your heart rate as measured at the end of the run or walk is lower 

Either of these can improve your VO2 estimate and both together will improve it the most. Looking at the formulas that people use to estimate VO2 max, of the two variables, becoming faster – even if this results in a slight increase in your heart rate at the end of the same distance – may be the most effective. As to how you go about improving those things, refer to ovekvam's suggestion of interval training and other activities that will see you working in a high heart rate zone. 

(Losing weight will also result in an improvement as that is another factor in VO2 equations, but only a very, very slight improvement if your times and heart rate remain unchanged. And as you age, a further variable, you need to improve your times and/or efficiency regardless in order to remain at the same estimated VO2 max.)

PS. You may need to go on a couple of shortish (10–15-minute) power walks without your dog. 

I read an article about VO2 max calculation and sport watches. The programmer explained how they tried to build a power curve at different heart beats, and calculating the VO2 max from that curve. The fitness age is just a table lookup based on what VO2Max is statistically typical for different ages and genders.

Based on your formula, my VO2Max should be 64, while it is currently around 54-55.

Yes, and that formula tells me my VO2 max should be 44 whereas Garmin estimates it at 39. :) I'm not suggesting that Garmin uses that particular formula, just wondering if their formulas do somehow take into account resting heart rate, which could account for a low estimate if the person's resting heart rate happened to be quite high. For example, my resting heart rate is usually around 60. But if it were, say, 70 then according to that formula my VO2 max would drop to 37 (and then, based on the table, my fitness age would increase).

Many thanks Thom. Very informative. 

Yes, under User Settings in the Garmin Connect mobile app there is a way to manually enter a VO2 max figure. 

More [ellipsis icon at bottom right of screen] > Settings > User Settings

Also in the web version: 
Profile icon at top right > Account Settings > User Settings 
(This gives you the option of entering separate figures, for running and for cycling.)

Thanks. Yeah, I remember those but they don't do anything. The V02 max numbers in performance stats seem to only change by function of a Garmin device. Manual input in settings does nothing here...