When attempting to update to SMG 10.5.2 from 10.0.3 you notice that the version is listed, however, you are unable to download it with the message "Version not available for download/install".
SMG 10.5.1 was a "gated" release. The reason for this is that in 10.5.1 we made changes to the kernel that require a direct update to this version prior to update beyond it.
Install SMG 10.5.1 and then 10.5.2 will be available for you to install. Alternatively, you can also do a fresh install of 10.5.2 with the ISO image. This, however, will require you to set the appliance back up from default settings.