Select Risk Register on the side menu and choose the risk you wish to add an update for.

Scroll to the bottom of the page and select Comments.

Here you can add your updates in the comments box.  Select Add Comment to submit it.  This will now appear below the comments box.

Comments will appear in chronological order as more are added.

Should you wish to remove a comment simply select the X icon next to the comment you wish to delete (you will only see this option if you have the relevant permissions to delete updates).