hi,
Through these files:
Product details:
/templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/default_addtocart.php
Category page:
/templates/YOUR_TEMPLATE/html/com_virtuemart/category/default.php
/templates/YOUR_TEMPLATE/html/com_virtuemart/category/flexv2.php
But i would suggest you to use CSS to modify the add to cart button stylization. Touching the PHP files might be tough thing to do.
For the CSS modification, this is the file you need to modify:
/templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexibleVM2Global.css
Regards,
Support