You can update the question set attached to the job anytime. Click on the Job icon in the right panel to view all jobs. Click the job I to Job's page. Click on the Job Details tab on the top right to view the job description, requirements, and other details.
On the right side panel, you would see the "Question Set" that shows the default question set attached to the job. You can click the drop down to view the other question sets that you have created. Choose a question set that you would like to attach with the job and click "Save" button that shows up. You can also create a new one by clicking the "Create New" button.