Go to Settings > Account. The Profile section of this page is where you can update both your profile image and username.

• To change your username, enter a new name in the Name field, then click Save at the bottom of the page.
  

• To change your profile picture, click the image thumbnail. In the popup, click Upload and select a photo file from your computer. Use the editing tools to adjust as desired, then click Save at the bottom of the page.