I emailed RevZilla, where I bought the headset. They said email #Sena, the manufacturer.
I emailed Sena. They told me to file a support ticket.
I filed a ticket. They told me, "We do not recommend replacing the battery yourself, as this may affect the performance of the unit. Batteries are not sold separately through our support channel. We recommend contacting your local dealer or the original retailer to check if the unit may still qualify for a warranty replacement options. [sic]"