You can delete your read stories from the library screen by following the below mentioned steps:

Step 1: Tap "Library" at the bottom left corner of app.

Step 2: In Library screen tap "Edit" at the upper left corner.

Step 3: After enabling the edit mode, tap the highlighted red icon.

Step 4: Delete option will appear right next to the stories, tap Delete to delete your stories

Step 5: When you have deleted your stories tap "Done" to disable the edit mode.