A full guide on how to implement and update the ads.txt file can be found here. You can also watch this 3-minute video tutorial explaining how to add ads.txt.
Ads.txt is a text file containing code that needs to be added to the root of all your domains in order to display and monetize our advertising. We request that you approve our ads by updating the ads.txt files of your domains with the lines provided here.
We will always notify you when we implement any update to the ads code, and will ask you to update your ads.txt accordingly.
For more information regarding ads.txt files, please see Authorized Digital Sellers or Google Support.