Just follow the steps below for your device:

Apple TV 3rd generation or earlier
  • The CBS All Access channel is automatically loaded during software updates. To make sure you have the latest version, go to Settings > System > Software Updates > Update Software. If there's an update, download and install it. 
  • To enable automatic updates, go to Settings > System > Software Updates and turn on Automatically Update. 

Apple TV 4th generation & 4K
  • Open the App Store and search for "CBS All Access". Select the CBS All Access app > Get. 
  • To enable automatic updates, go to Settings > General > Update Software and turn on "Update Automatically".