The site menu isn’t maintained using the built-in WordPress menu builder, but with a custom-built interface. You’ll indicate on your page that you’d like for the page to show up in the menu, then you can change the order of the menu items in a different area. Keep in mind that there is a limit to the width of the menu, so the number of menu items and menu item name length will need to be considered.
Step 1. The site menu will show only the top-level pages (parents). Sublevels (child pages) will drop down, so if applicable, you’ll first need to choose your page’s parent under Page Attributes in the page’s right sidebar. That will ensure it falls under the correct dropdown. (Learn more about site structure here):
Step 2. Further down the right side menu, you will see a box for Exclude Pages. Check “Include this page in the lists of pages” to add the page to the menu.
The menu will use the title of the page. If the name is too long or confusing, there is also a Rename Navigation Label box with a field to choose a new menu item name.
Step 3. Once you’ve published your page, the Tree View interface will let you reorder your menu items, if necessary. This interface is found on the WordPress sidebar > Pages > Tree View. You will see all the pages on your site regardless of whether they’ve been added to your menu, and the arrows can be clicked to reveal the child pages. To reorder the menu, just drag and drop the items (no saving needed), and the changes are instantly reflected on the front end. This interface is a bit buggy, so it may take more than one try.
The “Overview” link that you’ll see as the first item in your dropdown is the landing page for the top-level link that is clicked on in the top nav. Sometimes the link in the top nav is only a “bucket” for the links below it and there’s no content available for that top-level page. A blank page will still need to be created to add the text to the top nav, but it can be “hidden” by choosing “No” on the Page Overview Option on the page’s sidebar.