Hi Rien,
changing the template to change the style would be better to do it from your theme or creating a child theme.
You can add e default value to the field, in your WPC admin page at the backend, when creating a new field there is the options you can set.
I want to add that if you want to create custom templates in the product page here is a link in our documentation : https://altoswebsolutions.com/manual/woo-price-calculator/using-themes.
If you want to change the edit text in the cart page just change the content inside the file : wp-content/plugins/woo-price-calculator/admin/awspricecalculator/View/cart/edit-button.php.
Best regards,
Naidi
AT734