A couple of days ago someone in my family knelt on their Kobo-1 and busted the screen, so I checked how replaceable the battery is.
In theory the battery is replaceable, but you pretty much have to dissemble the entire thing while managing not to break the screen. So you would have to
1. Pry the front off
2. Remove the screen which is held down by scews.
3. Carefully pull the battery free of the tape that is underneath it.
4. Carefully remove the wire from the port.
5. Put the new battery in and put it together again so that it still works.
I wouldn't attempt it.