Story Remix is in Windows 10 Fall Creators Update version 1709.
Make sure your Photos app is updated by going to the Windows Store then updating it.
Also, you have to close and open the app to make sure the Story remix features are setup.
You can access the Story remix interface by doing the following.
Open Photos
Select the Photos and videos you want to use in your video
Click Add to (+)
Click Video
Give it a name
Click Create
You should then be in Story Remix
Drag and drop your photos and videos anyway you like
Add titles, filters, background music etc
Learn more:
https://www.groovypost.com/reviews/story-remix-windows-10-fall-creators-update-preview/