1. Go to Appearance > Menus.
2. Select the menu list that you wish to update.
3. Click "Select" button.
4. Select the page you wish to add from the list here.
5. If you can't find it from the "Most Recent" tab, click on "View All" to select the page from the list.
6. Tick on the page name.
7. Click "Add to menu".
8. Drag and drop the page to the desired place.
9. Click "save menu" once done.