I would like to update the quantity field whenever user select the order line

I try to update the field using the last line,

select: function() {
    $('tr.selected').removeClass('selected');
    this.$element.addClass('selected');
    this.order.selected = this.model;
    this.on_selected();
    this.order.selected.set('quantity', '123');
},

but I received this error.

TypeError: invalid 'in' operand attrs