When the partner has finished the work, they will submit the task. You'll get a status update that the task has been submitted.
Look in the task deliverables section. Use the view response option to look at their response, and look for messages in the Messages section. Use the chat feature to ask questions if you need to.
If you are happy with the work, use the validate section to accept the task. The task will then be marked as complete.
If you think the work needs redoing, use the validate section to request rework. Write a (polite) message about what you want them to redo, and submit the rework request.