Hi,
The previous/next buttons base themselves on the order you configured for each product in the main category of the current product.
So first, edit the product in the backend and check the categories setting of the product.
There, the first category listed is the one that will be used by the system.
Then, go in the products listing of your backend, and click on that category to see the list of the products of that category and set the "all sub elements" dropdown on the top right to "direct sub elements".
There, you'll have an "order" column where you can define the order of the products.
The next/previous buttons will use that order to select the products where the next/previous buttons will lead to on the product page.