Hi Ashvini,
Change requests are submitted to the CCB (or individual authorized by the change management plan). The CCB can approve, defer, or reject the change request. If two conflicting change requests were submitted, the CCB may approve one and reject the other. If two change requests were submitted that cannot be implemented simultaneously, then the CCB may approve one and just defer the other. Thus it would be the responsibility of the CCB (or individual authorized by the change management plan) to determine the priority of change requests.
See pages 115 and 120 of the PMBOK® Guide