Managed and Fully Managed Servers' backup feature lets you restore your server to the state it was in when you created the backup (more info). This backup is an image of your entire server, so you cannot restore individual files — you can only restore the entire server to its previous state, which overwrites any changes made to it since the backup was created.
You cannot undo a restore. Restoring your server permanently deletes all of the data on it and sets it up using the backup your created.