There are two ways to get into your Notification to edit it; the editing process and functionality is the same from that point on for both. 

Method 1 – Edit Directly

Navigate to the post in edit/secure mode, click on gear icon and choose Edit Notification:

Make required modifications and save/publish as usual.

Method 2 – SiteGovern

If you have created a Notification but not yet published it, this is the only way to edit it.

In SiteGovern, go to Recent Contents and choose the content type (shown on the right) from the dropdown.Select Notification and then, from the list of recent notifications, edit the one you'd like to change, using the wizard as usual: