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.
This opens the Edit review pop-up:
In case a review is in Pending status, the pop-up will name Approve review and have options to either approve or remove the review. A published review can not be removed using the edit review option.
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.
To save the changes, use the Update button for reviews in the Published state and the Approve button for pending reviews.