A firmware update method including receiving a first message from an update server, where the first message includes first data and a signature of the first data, verifying the first message, and obtaining digest data included in the first data after the first message is successfully verified. The digest data includes digest information of a differential installation package between a first installation package and a second installation package and at least one of, digest information of the first installation package, or digest information of the second installation package A second message is received from the update server and includes the differential installation package. The digest data is verified, and firmware of an embedded universal integrated circuit card (eUICC) is updated based on the differential installation package after the digest data is successfully verified.