Tap the preview to see how the video looks and sounds at its new speed. If you drag the slider to the right past the “1” value, it will speed up the video instead.
Tap Edit at the bottom of the video. The second slider at the bottom controls the speed. Drag the left and right sliders to select only the parts of the video you want to appear in slow motion. Anything outside of the two sliders will play at regular speed. Tap Done to save your edits.