You can update your notes for an outage of a monitor from the outage tab in monitor details page.
Navigate to the monitor details page you want to update notes for > goto outage tab (this tab will be available for all monitors),
Here you will the list all the outages that have occurred for the selected time period.
Click on any outage, a recent comment popup will appear. You can update the notes in that comment section.
The same will be reflected in the Updates section in the summary tab of any monitor details page
The same will be reflected in the Alarms details page, when the monitor is having a problem. You can view or update the comments from there as well. You can do so by navigating from Alarms tab > select the monitor to the updated > update / view the comments from there. Since this is an ongoing outage, you can view previous comments so that it will be helpful for resolving the issue.
Hope this helps. Let us know if you have any clarifications related to this.
Product Manager, Site24x7