Don't worry! This is a very simple thing to fix. You can update your first and last name at any time from our Personal Details & Notifications Preferences page. You can navigate to this page by clicking on your username in the top right corner of the page and going to billing settings.
From there, click on the personal details & notifications preferences link, and you'll be take to the manage account page.
The first and last name fields here will update your display name for the community and for any earned certificates of completion!