HDR10+ is a device feature option, in this document for Apple TV box (3rd generation), in combination with a TV that also supports HDR10+ (most notably Samsung). HDR10+ will then be supported for select content. There is no on-device conversion; in order to present HDR10+, it must be streamed as HDR10+ by the content provider. About 4K, HDR, HDR10+, and Dolby Vision on your Apple TV 4K - Apple Support.
On TVs that support both HDR10+ and Dolby Vision, then Dolby Vision will be preferred as the better quality option.
HDR10+ is supported from these content providers: Apple TV+, Hulu, Paramount+, Prime Video, and a few others. HDR10+ content providers - hdr10plus.org
Apple introduces the powerful next-generation Apple TV 4K (2022-10-18) - Apple Newsroom