Adding drop down quantity selector in Minimal Theme

[Minimal_User]

I'm using Minimal theme.
I want to change the current quantity selector to a drop down selector that will only display numbers up to the number of available inventory for each product.

So customers can't order more than what there are in stock or they know how much stock left at the same time.

I want to try the coding myself if it's not too complicated.
Can somebody please help me the coding for this change and where to put it?
Thank you in advance.