As soon as your writer uploads the final version of your assignment, your warranty period starts and lasts for 20 days unless you accept the order earlier yourself. During the warranty period you can request any number of revisions for your paper absolutely for free.
In order to do that you need to get to the order page (just click on the name of your order on the "My Orders" page). There you'll see three options: request a refund, send back for corrections, and accept the assignment now. Obviously, you need the second one.
If your initial deadline was in 2 days or less, your writer will have 24 hours to upload a revision.
If your deadline was in 3 days or more, your writer will have up to 3 days to do corrections, though, of course you can agree on a shorter time frame for a revision with your writer through chat.
If your writer fails to deliver a revised paper 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 writer thinks your instructions for a revision are completely different from the original instructions for your order, they may refuse to carry out any corrections.