Observer.php

 $customerLoad = Mage::getModel("customer/customer")->load(1234); // sample cust id
      $customerLoad->setWebsiteId(Mage::app()->getWebsite()->getId());
      $customerLoad->setStore(Mage::app()->getStore());
      $customerLoad->setGroupId(4);
      $customerLoad->setLatitude(123);
      $customerLoad->setLongitude(456);
     try{
                                   $customerLoad->save();
                                } catch (Exception $ex) {
                                    echo $ex;
                                }