For the mobile app, once you tap the library button at the bottom of the menu bar, you will see the stories at the top. Click on an individual story, and then you'll see it marked as "Saved" at the bottom of that menu bar. You can click on that box, and then just uncheck the story and it will be removed from your library.
For the website, you can hover above your profile picture and select "My Reading Lists." From there, select the respective reading list, select the story, and hover above "Add to Reading List" in the top righthand corner. You will see the checkmark on the list, you can just click on it and it will then remove the book from your library.