If you want to make an update according to the primary key you just need to use one criteria:

$c = new Criteria();
 
$c->add(MyObjectPeer::ID,123);
 
$c->add(MyObjectPeer::NAME,'ola');
$c->add(MyObjectPeer::CITY,'pepito');
 
MyObjectPeer::doUpdate($c);
 

and the query is:

UPDATE my_object SET NAME = 'ola',CITY = 'pepito' WHERE my_object.ID=123
 

Compared to the update query using Propel snippet which pass 2 criterias to the BasePeer:doUpdate method, here we pass only one criteria to the MyObjectPeer::doUpdate method. This method remove the index ID of the "set" criteria and use it for the "select" criteria.