There are 2 ways to change your thumbnail/splash screen from the edit page:
Select a frame from your video
Under VID SETTINGS, expand Thumbnails, then click Change Thumbnail. This will prompt you to choose a frame from your video. Just look for the specific frame you'd like to use as a thumbnail and click on it. You'll have the chance to confirm you want to make the change after clicking.
That's it! You'll see your new thumbnail appear inside the little box in the settings section.
2. Upload an image
Under the 'Change Thumbnail' rectangle, click on 'Upload'. Choose an image from your computer. The image will be uploaded and automatically stretched to fill in the whole video size. Keep this in mind when you're choosing your image (you might want to use one with the same aspect ratio as your video).
Note: If you are unable to upload your thumbnail, remove the special characters, i.e. parentheses ( ) from the file name.
You can also choose to use a different thumbnail for mobile. If you don't, the desktop thumbnail will be applied by default.
Another awesome feature is the exit thumbnail. You can learn more about it here.
Don't forget to make your changes live by clicking on the 'Publish' button to the top right of the video.