As soon as your expert uploads the final version of your project, your warranty period starts and lasts for 20 days unless you accept the project earlier yourself. During the warranty period, you can request any number of revisions for your project absolutely for free. 

To do that, you need to get to the project page (click on your project's name on the "My Projects" page). There you'll see three options: request refund, request corrections and accept the project. Obviously, you need the second one.

The duration of the said correction will be visible on the project's page at all times:

If your expert fails to deliver a revised project in time, you'll see an option to request a refund as soon as their time frame for a revision elapses. 

Keep in mind that if your expert thinks your instructions for a revision are completely different from the original instructions for your project, they may refuse to carry out any corrections.