Add this code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Dessky Snippets plugin. Please don’t add custom code directly to your parent theme’s functions.php file as this will be overwritten completely when you update the theme.

Set the starting value, maximum value, minimum value and increment amount.

/**
 * Adjust the quantity input values
 */
add_filter( 'woocommerce_quantity_input_args', 'jk_woocommerce_quantity_input_args', 10, 2 ); // Simple products

function jk_woocommerce_quantity_input_args( $args, $product ) {
	if ( is_singular( 'product' ) ) {
		$args['input_value'] 	= 2;	// Starting value (we only want to affect product pages, not cart)
	}
	$args['max_value'] 	= 80; 	// Maximum value
	$args['min_value'] 	= 2;   	// Minimum value
	$args['step'] 		= 2;    // Quantity steps
	return $args;
}

add_filter( 'woocommerce_available_variation', 'jk_woocommerce_available_variation' ); // Variations

function jk_woocommerce_available_variation( $args ) {
	$args['max_qty'] = 80; 		// Maximum value (variations)
	$args['min_qty'] = 2;   	// Minimum value (variations)
	return $args;
}