Greetings,
If you wish all the items to appear in the top menu, rather than categories on the left sidebar, you may set the Template of the items in question to Simple.
Greetings,
We are taking the first category for the product through the nopCommerce GetProductCategoriesByProductIdAsync method in the Category Service.
NopCommerce first orders the categories returned by this method by display order and then by id of the category.
You can either change the display orders of your categories or modify the nopCommerce code.