You will need to be in the SharePoint editor group before you are able to do this:

  1. Due to Maddocks still using the hybrid mode (Classic and Modern), you will need to do this twice
  2. On the modern page, click edit next to the navigation and add/remove the page (depending on the request) there
  3. To remove, click the three dots to view more options and select remove:
  4. To add a new page, hover your mouse to the location where you would like to add a page and then click the + sign
  5. Once you have finalised the change/add the new URL, don't forget to click save at the bottom of the menu list
  6. On the classic page, go to this link to Intranet - Mega Menu - All Items (sharepoint.com) 
  7. Find the parent page and then expand the contents
  8. Click any of the item and start editing the content on the right-hand side navigation or click remove if you need to remove the content, it has an auto-save feature and there is no save button.
  9. to add the link to the intranet megamenu, click New button and select Item.
  10. Fill the required information on the right hand side pane, click save