Vo2 not updating

First force restart the Watch

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.

Then, although it may seem irrelevant, Force Restart the paired iphone   Force restart iPhone – Apple Support (UK)

If is is still not updating (it can take a week or three).

 Unpair and erase your Apple Watch

and then

Set up your Apple Watch

 If no better you may want to unpair then set up without restoring the backup.

You may also want to make sure the software on the watch and phone are both up to date.

If you've completed that troubleshooting and the issue is still not resolved, Contact Support.

Click here > Contact - Official Apple Support

or phone numbers 

Click here > Contact Apple for support and service - Apple Support

If you mean cardio fitness, have you done outdoor runs or outdoor walks since then? Only those two activities will update this reading.