How to implement and update the Ads.txt guide can be found here. Or you can check out this 3-minute-long video tutorial on how to add the ads.txt.
The ads.txt is the code that you will have to add to the root of all your domains to approve our advertising to show and monetize on your domains. Hence, we kindly ask you to update your domains' ads.txt files with the lines in this link.
Note that whenever we have an update on the ads code, we will let you know and we will ask you to update your ads.txt accordingly.
More info on the ads.txt: Authorized Digital Sellers, or Google support.