Add the following lines using the Code Snippets plugin. You can add the following lines inside your theme's functions.php file
remove_filter( 'pvt_print_plus_minus_qty_field', 'pvt_display_qty_plus_minus_button', 10, 1 );
add_filter( 'pvt_print_plus_minus_qty_field', function( $value ){ return woocommerce_quantity_input($value); });
Note: A child theme should use if you want to add the above lines inside your child theme's functions.php file