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 Attachments.

Here you can upload a document from your device by using the Add file option.   The attachment will now appear below in the table. 

Attachments will appear in chronological order as more are added.

Should you wish to remove an attachment simply select the X icon next to the attachment you wish to delete.