Actually, you can’t. Why not? Because the First/Last Day Highlight Reel is automatically created by pulling the first & last day photos you upload in each individual grade album.
To add or change a first or last day photo, navigate to the child’s profile. Tap the grade album you wish to edit. Select First Day or Last Day above the photo placeholder. Tap Add [First or Last] Day of School Photo on the placeholder. Follow the prompts to choose a photo from your gallery or use your phone’s camera to snap a pic.
To edit the photo, tap the three dots [...] in the bottom right of the photo, then tap the Edit button. From here, you can modify the photo using the pinch & zoom gesture with your fingers or change the orientation using the layouts below the image. To change the photo, tap Change Photo on the bottom left. Any changes made to the photos within the grade album will automatically update the photo in the First/Last Day Highlight Reel.