Recently we have posted a tip about how to quickly create a table of contents. However, sometimes it’s necessary to update a table of contents manually.
In the Table of Contents file, press Return after the last item in the list and a new item will be created. Then holding the ⌘ (Command) and ⌥ (Option) keys drag a document or group to that new line to insert a link to the dropped item. To allow seeing the table of contents as well as other documents, you’ll obviously need to use a view that allows it, e.g. View > View as Three Panes, or have another window open.
This short screen movie shows how this works: Update Table of Contents.