For more info on Free Up Space, also read this article.
To free up space for a specific photo or video, simply tap on the photo to open in fullscreen, and expand the top-right menu. If the photo is stored locally, and the photo has been backed up, the Free Up Space option will be visible.
To delete the local version of the photo, choose Free Up Space in the expanded menu. Then confirm the delete by pressing Free Up Space.
Remember to remove the recently freed up photo from the "Recently deleted album" in iOS Photos app as described at the bottom of this article.