In a scenario where multiple devices are configured with active and standby roles, it is generally recommended to upgrade the standby device first. Upgrading the standby device ensures the least amount of disruption to active services and allows for the recovery process if any issues arise during the upgrade. Once the standby device is successfully upgraded and verified for stability, other active devices can be upgraded one by one to maintain service continuity. Therefore, Device C, which is in standby mode, should be upgraded first in this case.