Open the Photos app
Select the photos you want to use
Tap the Share button
Scroll and tap Slideshow
To save it as a video, open the Memories or use iMovie
Open the iMovie app
Tap Create Project
Tap Movie
Select your photos
Tap Create Movie
Arrange the photos in the desired order
Add transitions, text, or music if needed
Tap Done
Tap the Share button
Tap Save Video
Choose the video quality
Wait for the video to save to your Photos app