Change the order of product variations on the "add to cart" form
Similar to Drupal 7, Drupal Commerce for Drupal 8 respects the ordering of attribute fields on the product variation form when rendering them into the Add to Cart form. To find these options, you'll need to navigate to Administration >  Commerce > Configuration > Product variation types and select the Manage form display option from the Operations dropdown for the variation type in question. Reorder the fields there, and they'll appear in the new order on the Add to Cart form.