You can add Comments or Updates to your Incidents directly from the Incident List or within the Incident Details Page.
In the Incident List, click the three dots on the right hand side of the Incident tile to bring up the option to Add Update to that Incident. Simply type your comment, @ mention any other user if necessary, and click 'Add Update'. This comment will also be recorded in the activity history for the incident.
Alternatively, you can add comments at the bottom of the Incident Details page, straight into the History section. Simply type a comment and add it. This activity log will record all changes and comments applied to an Incident, from creation to completion.