Modify 

If you want to modify a topic or a definition then press the "pencil" icon at the end of the definition's line. 

Then you can see the same fields as you used during creating the definition. Modify the definition or the topic and then press "Save". 

Delete

If you want to delete a topic or a definition then press the trash icon at the end of the definition's line. You will get a confirmation question where you must press "Yes".