From the "Preview Your Video" screen after Step 3, tap "Save" in the top right corner.
After selecting a file size to upload, tap "Save."
Scroll to the bottom of the "Share Your Video" screen and tap "Change Thumbnail."
Scroll through the video to find a thumbnail image. Once you're satisfied, tap "Save."
Give your video a unique name and then tap "Save" to upload the video with the new thumbnail icon.