Alloy ACS update existing custom object

Hi,

I use Aaron Saunders ACS example https://github.com/aaronksaunders/ci.alloy.acs.books/tree/master/app

Collection fetch works great. Please could anyone tell me how to update existing object in the cloud. Calling "save" will add new object, not update it.

I need to fetch the Collection, get 1 object, update fields and save it to the cloud.

Thanks