There are a few different ways to leave reviews for freelancers,
1) From Your Contracts Dashboard
From here you are able to click on the 3 dots under the 'Action' header, and select 'Review 'Freelancer'
2) From Your Conversation With a Freelancer
You can also leave reviews directly from the conversation with a freelancer by selecting 'GIVE REVIEW' on the bottom right hand side.
You will then be asked to review the freelancer out of 5 stars, and invited to leave some comments