How can I upload a DVD to my Apple TV?

You can’t. The Apple TV box doesn’t support “uploading” any content to it. All accessed content is streaming from somewhere else.

To play DVDs on a set-top DVD player, connect it directly to your TV. The Apple TV box is not involved.

Select DVDs may offer a redeem code for a “digital copy”. If that digital copy supports iTunes, then you may add the title that way to your Library of purchased iTunes Store movies. Redeem digital copies of DVDs or Blu-rays in the Apple TV app or iTunes for Windows - Apple Support

If this DVD is a non-commercial disc, like a home video, then you could try to convert it to .mp4 on a computer, and then home share it locally to Apple TV. Use Home Sharing to share media from a computer to other devices - Apple Support.

you can't

you will have to use your tv remote to choose the dvd player as active input source rather than the appletv when you wish to watch a dvd