Regarding update API

I am using "Omnisend PHP-SDK". I am successfully implemented post API and get API but I am not getting how to use put API for update so could you please share the syntax of update API. 

I can fetch the contactID using get API. For example:
 $contacts = $omnisend->get(
   'contacts',
    array(        
        "email" => $email 
    )
);

Please let me know the update ( put ) API.