In case your client left a positive review for you and you want to reply something nice or to thank your client, you can do so on the page with all your reviews.
To get to that page you need to go to your profile page first.
Scroll down to your reviews and click on the link "See all reviews".
On this page, you can check all the reviews left for you by all of your clients. You can respond to your reviews by clicking the COMMENT button.
If you got a negative review, you can either simply comment on that as well or to send a petition to the administration to remove that review if you believe it is false or unjustified.
It is also possible to send a petition against a negative review straight from the order page if that review was left for a completed order. If you got a negative review for an order that you dropped halfway to the deadline, this page is the only place where you can comment on or petition such a review.
You can check more information about petitioning against negative reviews here: