The right menu is a widget. The top menu is a menu. To change a menu, go to
Dashboard > Appearance > Menus
I’m not sure what your menu is called. I think its “Primary”.
In the left panel on that page there will be several sections. Locate the Product Categories section. If this section is not showing, enable it in Screen Options.
The section will have a little down arrow – click it to expose your product categories. Check the categories you want to appear in your menu, and click add to menu.
These product categories will be added to the bottom of the menu on the right side of the page. Drag and drop them up and down to put them in the order you want. Menu items can also be dragged and dropped slightly to the right to make them sub-menus.
When you are done, click Save menu.
If you want to do it manually, the link format is:
https://remmymeggs.com/store/product-category/books/
not
https://remmymeggs.com/store/books/