Select "settings" from the drop-down on the top right, and then select the green "Edit Creator Page" button.
From there, scroll down until you see "Upload photo for your page." Make sure to use a photo that will work stretched out (a resolution of 1600x400 works best).
If you'd like to remove your cover photo, you will need to select a replacement photo.
Note: We're slowly rolling out an improved way to edit your creator page. If you are one of the lucky users to be testing out the new feature, you can upload your cover photo directly from your creator page by hovering over the top of your page.