To unsubscribe or add a new 'Notify Me' click the link that appears in the bottom on an email received from the notify me system.
There are 2 boxes on the modify page.
The first box is to delete your subscriptions, called "Saved Entries". To delete a subscription check the delete box for each entry you wish to delete AND THEN click the "Delete Previous Entry(s)" button.
The second box "New Entry" is to add a new subscription. Choose an event type and select a library or all libraries and click the "Save" button. This subscribes you to a new "Notify Me" subscription.
Editing a subscription is simply deleting the old entry and adding a new entry.