Select the "detail" button of the exam course in your homepage. The blue button at the top of this page enables you to schedule a retake exam for each module not passed (and buy the retake f applicable). A waiting period of 15 days needs to be respected; for example, if you failed your exam on September 1st, your retake reservation will need to planned from September 16th on.
You will be informed by e-mail when your L&D coordinator has booked your next retake session. Alsoin this case the waiting period of 15 calendar days applies.