Add code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file as this will be wiped entirely when you update the theme.
Set the starting value, maximum value, minimum value and increment amount.
https://gist.github.com/woogists/74e57feaef6e3d5f38d9c08ac92d8712
If you are looking for a little more power, check out our Min/Max Quantities extension!