Hello George, I'd be happy to offer some guidance on this as we get quite a bit of screentime on upgrades here in support! There are a couple of ways you can go about this: 

1. In place upgrade of Windows Server OS, followed by upgrade of SmarterMail to the latest release. 

2. Migrate (with existing version) to new server, then perform upgrade to SmarterMail. 

On the SM upgrade itself, moving to our latest release will involve a conversion process for the system/domain/user level configuration files, and an entirely new mail index will be generated for users so that they leverage the newest functionality. As such, I strongly recommend a full backup of the SmarterMail environment prior to getting started: 

Once backed up, you can upgrade directly to our latest release without having to stop off on version 16 first. I've included major version upgrade instructions here as well:

Once upgraded you can monitor the domain conversion process by navigating to your server's newly available convert-status page. I've included an example of how to access that here: 

This page will monitor and display current details about the domains in the conversion queue, and typically completes in just a few minutes, though this can take considerably longer depending on how many domains/users are present on the server. Once this finishes all users should be able to log on again and check mail as usual. At that point the user reindex process will kick off and can take a few days to complete depending on available server resources and the type/size of mail content and indexes. 

Please let me know if you have any questions or need further assistance. I'm sure users here in the community can likely add caveats and tips/tricks as well so stay tuned! Have a great day!