Change default product category.

functions.php

add_action( 'init', 'wc_ninja_update_default_category_once' );
function wc_ninja_update_default_category_once() {
    // 20 = new default category ID
    update_option( 'default_product_cat', '20' );
}

WooCommerce 3.3.2 will have a way to set a different category as the default. Until then, here's a quick workaround.

1. Add this PHP snippet to your site, perhaps use the Code Snippets plugin mentioned here:
   https://calebburks.com/correctly-add-custom-code-woocommerce/

2. Replace '20' with the category ID that you want to become the new default category.

3. Refresh the page, then delete this snippet. (only needs to run once)

4. If you want, you can then delete the previous default / "uncategorized" category.