To replace cover images, simply navigate to "Upload/Preview Files" from your Title Details page.
Next, browse your computer and attach the new image.
If it doesn't update right away, it's possible you're seeing a cached version of the file. To troubleshoot, try refreshing the page, clearing your cache, or viewing the title record on a different browser. You can also use the "Preview" button on the Title Details page to see your title record and cover image the way a NetGalley member would see it.
Please note: If you replace your cover image, your cover vote count will reset to 0.