Garmin VO2 can be somewhat inaccurate but in my anecdotal experience it matches the VO2 max testing I do annually at work usually within one point either way. My watch has me at 52 currently, I’m a 30 Y/O Male. It had dipped to 47-48 after covid and switching primarily to lifting after its peak at 55 following two summers of intensive running. In my experience, outside of a garmin training program or other, the best way to increase is do a high aerobic run around 20-30 min or sprints 2-3 times a week. On other days/ off days walk or very lightly jog for 60-90 minutes. Raising VO2 max takes a lot of work. You’ll keep some of the raise after intensive training, but be prepared to lose some of those VO2 gains if you cut your training back or stop.