There is a mandatory 14-day waiting period after a failed attempt before you can apply for a retake. After that, log into your professional account (blue banner) and locate the retake application under the My Testing tab.
Each candidate has four attempts total (including the initial attempt) within a one-year eligibility window. For professional credentials, that window begins when your application was approved. For associate credentials taken through the education pathway, it begins when you completed the qualifying course.
Your score report for a failed attempt will show how you performed in each domain of the exam. Use that breakdown to focus your studying, cross-referencing it with the Job Task Analysis (JTA) for your credential on the NABCEP website, which shows how much of the exam each domain represents.