So, one more question (in addition to the ER code): where is your Update code called from?
You can also use event receivers to validate the data that the user wants to change.
This walkthrough is based on the first one where I have a custom list with planets of our solar system.
Are you able to debug your Event Receiver when you modify an item in the list?
It is nice to update Title in Item Adding and Item Updating events since extra Updates are avoided (without Disable Event Firing), and the "Edit Properties view" will already have the Title filled in Some code example to update title based on filename. For working with Title in Lists, ["Title"] needs to be used.
Share Point Online Event handlers are deployed as Remote Event Receiver (RER), which uses the client-side object model (CSOM).