Apple TV Siri Remote Battery
Can I get a replacement battery or remote when my siri remote won’t keep a charge?
Apple TV HD, tvOS 15
There is a battery service for Siri Remotes. Contact Apple or an Apple Store/Authorized Service Provider near you. Replacement of the whole unit seems standard practice.
Apple TV repair - Official Apple Support
↳ Battery service for your remote
Apple TV Siri Remote service pricing - Apple Support. Depending on warranty status, there may be a fee.