This has happened to me, I did the update and it died withing 4 hours I charged it and it died again. The next day I left it at home and the battery was fine as soon as I put it on it drained so I don't know of its because it drains when connected to my phone.
UPDATE AFTER I RESET AND CHARGED MY WATCH ... I used my watch as I normally do from when I took it off the charge ( at midday ) and not @ 7.10pm it's still on 83% maybe it's just luck but I would DEFINITELY recommend a reset