While sending the work file, there might be some issues left in the work, or some parts of th work that you may want to get imrpoved. In that case, You will have the right to ask for a revision.
On the contract page, there is an option available next to the work that the contractee submits. That whether you accept the work, or send a revision. Just click on the Send Revision and tell the contractee about the changes that you want- in the chat section.