Yes, you need to repost a job, once you make changes to it. The easiest way to do this, is to select the jobs using checkboxes under Jobs > Management, and select "Post to Job Boards" under the Group Actions drop down on the top of the page. This will launch a wizard that will allow you to choose the job boards where you wish to update the job postings to and complete payment.
To refresh each position individually to job boards, you can click the job position. Go to the Sourcing tab, and under "Active Job Boards", click the Refresh checkbox for the job boards where you wish to update and click the "Update" button.
NOTE: Please remember, that if your job position is expired, you may be required to pay to update the job posting