A store admin can edit any product review on the Reviews page in the store's Admin area (Catalog -> Reviews).
To edit a review, click on the Pencil icon on the review line.
The Edit review pop-up will open:
If a review is in Pending status, the pop-up will name "Approve review" and have options to approve or remove the review. Keep in mind that you can not remove a published review this way.
A store admin can change literally every line in the review details, including the rating itself. It is also possible to reply to a customer using the "Add response" link at the bottom.
Use the Update button for reviews in the Published state and the Approve button for pending reviews to save the changes.