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 Home page choose Listings, View/Revise Your Listings. Below the appropriate listing number, click Revise, 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.