Most file formats get a thumbnail preview generated automatically when uploaded to Image Relay, although there are a few that do not (AutoCad files are one). Or, perhaps you want the thumbnail to display a different page from the uploaded PDF, or different frame from the uploaded video that is previewed.
Locate the asset for which you wish to update the Thumbnail Preview, whether a zip, a video file, or .psd/.ai.indd, etc.
Hover over the asset and click Details.
Click the options ellipse in left corner, then Update/Replace Thumbnail.
In the pop-up that appears, click Browse for File then Replace Thumbnails.