Okay I got this watch last year (August) and it's suppose to have a 5 year battery (life) great...but it died last month

so I looked at the back of the case and sure enough it's a screw on/off type with tools I don't have, namely a rubber ball. You can order a special decagon wrench but you have to buy them in sets and they are expensive and I only need one size and not sure which size exactly is it

so I ordered a rubber ball (and new batteries).
They finally came in the other day (after a battery mixup/non-delivery) so I went to work

you can get a cheap Chinese rubber ball that probably will work but I went for the established brand Bergeon (twice the price)

the decagon back

ball on back

now at first I tried removing the back cover on the blue silicon mat you see made for stuff like this but it wouldn't work/screw off so I put the watch in my left hand (palm) and held the ball in my right hand and I was able to screw the back off with a little force and pushing and squeezing the ball (it has a nozzle for air hose for add/removing air to increase decrease pressure inside the ball if needed to make it harder or softer and it helped I made it harder/fuller).
Success! got the back off (sorry I didn't take pictures of that) and replaced the battery (the old one was 1/2 power of a new one) and replaced the back cover and screwed it back on with the ball until it could turn no more and voilà it works. Hopefully it will last much longer!