Delete photos from phone (but keep them in Google Photos)

With Google's Back up & sync, if you delete a photo from your phone in Google Photos, the photo will be deleted from any where. In case this is what you intend to do, here're the steps

1. Open Google Photos
2. Click the hamburger button at the top left corner, or swipe right from left edge of screen
3. Select "Device folders"

You know how to do the next.