Any aggregate ratings (marked by the large aqua bubble) you apply in the Aggregated Evidence & Ratings page (AE&R) remain static until you change them. If you wish to preserve your AE&R ratings at a certain point, you can take a Snapshot of the ratings to capture the ratings. Using the Snapshot feature is covered in Creating and Using Snapshots.
In order to remove a prior rating, click the rating bubble to deselect it.
In order to update a prior rating, click a different rating bubble to change your rating.