how to update my Apple TV via computer?

Which Apple TV model do you have? If you have an Apple TV 3 or older, you may be able to connect it to your computer. If you have a current model, you can't.

Identify your Apple TV model - Apple Support

Apple TV is a streaming device. It needs to connect to a home network (Wi-Fi or ethernet) to function as intended.

To erase and restore the system software (different from regular updating) through a computer: Restore your Apple TV - Apple Support