There are a couple of methods you can use, but definitely with some caveats.
1. If the Mac is enrolled in DEP and is running 10.13.4 (or later) you can use the Software Update command on an individual client page in the Meraki dashboard. It should appear in the same grouping as remote desktop/remote lock/etc on the client page. This is super manual however, I don't know of a Meraki-supported way to do this in batches.
2. You can use startosinstall as part of the macOS Mojave installer, however Meraki would also need to support this in some way to remotely push out. You could use an app management tool like Munki to force an update with startosinstall, as an example.
https://support.apple.com/en-us/HT208020
