Start by clicking on the photo icon labeled "Media" in the toolbar at the bottom of the page to access your media center. There are two ways to do this:
1) The first is to upload your preview and full video to the media center.
On the full video, click on "Manage", then expand "More Actions".
Select "Create New Preview", then select the uploaded preview to add it as a custom preview.
2) The second method is to go directly to "Create New Preview" following the instructions above. Click to play the video.
Then move the sliders to select a portion of your video, and click the "Generate Preview" button that appears.
Once the custom preview is rendered, select it from the "Create New Preview" menu to use it as your custom preview!