Get current exchange rates

Hi, I'm using the script below to get total cost of a event, considering the system default currency (USD), but the exchange rates aren't updated.

How can I get this rates updated?

var totalCost = Class.create();
totalCost.prototype = {
       initialize: function(mrkevent) {
               this.mrkevent = mrkevent;
       }, 
       getCost: function() {
               var cost = 0;
               //Get the total cost of all equipment for this event
               var equipment = new GlideRecord('x_73586_marketing_equipment_request');
               equipment.addQuery('marketing_event', this.mrkevent);
               equipment.query();
               while(equipment.next()){
                       cost = cost + parseFloat(equipment.cost.getSessionValue());
               }
               return cost;
       },
       type: 'totalCost'
};

Thanks!