Use had better (negative: had better not) and the verb in brackets to complete the warning in the negative: The firmware update is only half installed. You ___ the controller, or its memory may be corrupted. (restart)

Բացատրություն

Had better not restart follows the requested negative frame and gives a forceful warning about the concrete risk to the controller. Must not restart can express a prohibition, but it does not use the construction prescribed in the prompt.