If a table of contents is created automatically from the headings in a document, Word can update the table of contents when those headings change. (Learn more about creating a table of contents.)
Click References > Update Table.
Or in Word 2016 for Mac.
In the Update Table of Contents dialog box, choose Update page numbers only or Update entire table, and then click OK.
Important: If the table of contents was created manually (in other words, if it wasn't created automatically from the headings), Word won't be able to update it for you. Instead, you'll need to manually type your changes in the table of contents.