Method for updating key in DCATV conditional access system

The present invention properly updates a key in various situations in order to secure liability of a service in a conditional access system for a digital cable television service. A method for updating a key in a conditional access system of a digital cable television service includes the step of receiving a request of changing service from a point of deployment (POD) module of subscriber having bidirectional or simplex receiver, changing a service at the head/end and sending an entitlement management message (EMM) having a changed key from the head/end to the point of deployment (POD) module.