A method for updating an operating system (OS) comprises: receiving, by an embedded universal integrated circuit card (eUICC), a restart instruction sent by a local profile assistant (LPA) of a terminal device and used to instruct the eUICC to perform a restart operation; sending, to a modem of the terminal device, a first initialization request used to request the modem to control restart of the eUICC; after being restarted, receiving, by the eUICC, a plurality of OS element data packets that are sequentially sent by the LPA; after receiving a part of OS element data packets in the plurality of OS element data packets, parsing, by the eUICC, the OS element data packets received by the eUICC, and installing a first OS based on a parsing result; and deleting, by the eUICC, the first OS element data packet.