Hi Guys,
I would like to share my experience with update.
We have also tried to upgrade SMG 10.6.4-3 to 10.6.6 but it was failed.
We have tried to upgrade twice in SMG GUI mode but everytime SMG was unable to complete download of update packages and at the last stage of download it auto logged out and lands to the login page.
I have cleared update data and retried but no luck.
also tried it with command line but in command line it was not initiating download, finally I have tried local install but still not succeeded.
Finally I came to know that 10.6.4 & 10.6.5 are having update issue, please check the below article
https://support.symantec.com/us/en/article.tech251050.html
I have raised ticket with Symantec to provide solution to this because the fresh installation via osrestore is not the solution where one has to recreate policies and settings from the start.(due to limitation of SMG backup restore to the same version)