The battery is not "user replaceable." Searching YouTube, I see some have attempted it. I suggest reaching out to Garmin Support first before attempting anything like this.
I have replaced the battery in one of my vivoactive 3 watches. It's not really difficult to do if you are generally handy and are comfortable with taking the unit apart. However, the replacement batteries are not original Garmin branded products, and I found that I really didn't gain any significant increase in battery life compared to the 5 year old original battery....... maybe an additional 1/2 day.
