Buy a new battery and take it to your local phone repair shop. Or do it yourself if you feel confident with a bit of light soldering.
https://www.ifixit.com/Teardown/Kobo...eardown/162028
BTW, the first step in that guide seems unnecessary for our purpose. At least I was able to take the back off my Forma without removing the fingerplate. I didn't actually replace the battery on mine because both are still doing surprisingly well. But that's my backup plan.
