If your listing has expired, you may or may not be able to change it back to active, as determined by your MLS.
If this is allowed, from the Listings pull-down menu, choose View/Revise Your Listings. Next to the appropriate listing number, click the Revise icon , and then choose Status Information. In the Status field select "Active," enter or clear the remaining fields as necessary, then click Submit.
If this is not allowed, contact your MLS staff for assistance.