Hmm that information does seem to be missing from the guide.
This will get you going though.
// Print the existing models
print_r($object['models']);
// Change the models
$object['models'] = array('islandora:collectionCModel');
// Print the new list of models
print_r($object['models']);
// Check if it worked
if(isset($object['models']['islandora:collectionCModel'])) {
echo "Successfully changed the content model";
}
// Remove the model
unset($object['models']['islandora:collectionCModel']);
if(!isset($object['models']['islandora:collectionCModel'])) {
echo "Successfully removed the content model";
}
Nigel