We need to provide the information of service account when installing Cloudbees Flow server or agent in Windows. This service account is used to start the Cloudbees Flow server or agent. More details is available in Running an Express Server Graphical User Interface Installation. The following is a screenshot of the Cloudbees Flow agent service:
You need to change the password of the service account for some reason. This will cause the configured service account information in the Windows service to be invalid and thus cause the Cloudbees Flow service fail to start.
You need to provide the new password in all relevant machines to update the Cloudbees Flow service.