Look on the bottom of your Apple TV for the model number (e.g. A1625) Match it up with the list of Apple TVs on Apple’s support document or with the list below; Alternately, ...

Currently, the last Apple TV version still supported is A1625, or the "4th gen" ("Apple TV HD"). I'd guess it might not get much support anymore with its old A8X CPU, as the equivalent iPhones don't.

Model numbers are A1218 for 1st generation, A1378 for 2nd generation, A1427 or A1469 for 3rd generation, A1625 for Apple TV HD, and A1842 for Apple TV 4K. Where you'll find your Apple TV model number.