Adding $product.minimal_quantity to quantity field by Up or Down button

[RiesjardVV]

Dear Prestashop forum,

does anybody know where i have to do adjustments in javascript when in PS 1.7 i want to chenge behaviour of Up and Down button on product detail page.

By default, the products quantity wanted = 1 or $product.minimal_quantity when this is configured for specific product.

Let's say $product.minimal_quantity for product A = 10.The products wanted on product detail page = now filled with value 10.

I want that on clicking ^ Up button the products wanted is raised with the value of product.minimal_quantity so -> 20 -> 30 -> 40 etc etc

Anybody who can help me out, willing pay if needed.

Grtz RIchard