Go to your Dashboard. In the section Your Completed Courses & Claimed Certificates, please click on Redeem Certificate, as per the screenshot below:
If you have completed more than one course, you can choose which course you would like to redeem your purchase against.
It is important to note that digital Certificates can only be redeemed against completed Certificate courses, and digital Diplomas can only be redeemed against completed Diploma courses.
You can download Certificates from Your Completed Courses & Claimed Certificates section: