Once you view your live photo follow these steps:
- select edit (upper right)
- under the photo, you will see a scroll
- move the scroll from left to right to find the photo you want
- select Make Key Photo
- tap the yellow live button (top middle)
- select yellow Done (bottom right)
- if you make a mistake and want to go the Live Photo, simply select Live Photo again and repeat the steps.
Embedded below is a 43 second YouTube tutorial showing how to accomplish this.