First turn on live photos on your camera's phone by pressing the button at the top between HDR and the timer.

Next, take a photo of some moving water.

View the photo then tap edit and swip up.

This reveals the effects, scroll the effects to the right until long exposure appears.

Tap the long exposure icon to convert your photo then save. Job done, you've now blurred the moving water while keeping the surroundings in focus.

This demo was created using the first generation iPhone SE.