There are two ways to revert your site to an earlier version.
Use the Revision feature
To ensure the ability to revert to an earlier version of your content you can use the revision option for a page. This is explained in more detail in the article Using the Page Revision Option.
Ask to have your site restored from a backup
Drupal Cloud sites are backed up nightly. If you want to restore your site to the state it was in on a previous day, send a request to firstname.lastname@example.org, giving the your site's url and the date of the backup you wish to restore. Backups are retained for two months.