Yes. Even if you already rated a book, you can always return to it and change the rating or / and post a review. This can be done from the main page of the book (where the rating stars are present). To change the rating or / and add a review, click the stars. A new windows should appear in which you can select the stars for rating and write the review. After you did this, press the Save button and ADD review button. This can be done as many times as you want.