Abstract: The embodiment herein provides a method for securely transmitting a firmware update image to a device using a key management system. The key management subsystem includes a cellular modem. The method includes (i) configuring a SIM of the cellular modem to update a public key of a server using a key manager module of the Subscriber Identity Module (SIM), (ii) enabling the SIM to receive an encrypted key package from the server, using the cellular modem, (iii) processing the encrypted firmware update image that has to be transmitted to the device using the SIM and (iv) transmitting the decrypted key package to the device to enable implementation of the decrypted key package into the device using the SIM.