Insiders today are receiving an update to the Microsoft Photos app, which now includes the ability to create a slow motion video from any existing clip.
Of course, the Windows Camera app has supported recording slow motion video for some time now. However, if you've ever recorded a video and wish you had turned on slow motion, you can now make it so.
You should keep in mind that slowing down a video through the Photos app is not going to produce the same result as if you had recorded it in slow motion. A video is made up of frames, and smoother videos have more frames per second (fps).
Typically, a slow motion video would be recorded at a higher frame rate. That way, the video can be slowed down and still be smooth. Your phone most likely records regular video at 30fps, or even 60fps, but most phones will record slow motion at 120fps. Resolution is often downgraded to achieve this.
To grab the update, make sure that you're on an Insider build and head over to the Windows Store to check for app updates.