I would like to change the Product Details 'Add to cart' button with sample one, not this green ajax button of Flexible. How to do this, in which file?

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