Most phones don't have replaceable batteries these days. I don't think any smart watches do. The problem is to get the power density within the form factor they can't use off-the-shelf batteries and replaceable sockets so the battery is basically a chip soldered to the board in the sealed housing.
To be blunt with what we've all been saying, there's nothing you can do with that watch. It is functioning as designed: it parted someone with their money. That it even turns on after the money makes it's way to southeast asia is irrelevant to the maker or seller.
