The International Unlocked version comes directly from Samsung, so you would, in theory, get updates even before US carrier versions (in short, Google releases the update, then manufacturers release after a while for their factory unlocked versions and give that to carriers, and then carriers take that and modify it and then release for their custom versions, if at all).
That being said, system updates sometimes are rolled out in stages. It could depend on the region your phone was manufactured/intended to, so while an update could be out for Europe, for instance, you'd have to wait until your region was updated if your phone was meant for, say, Brazil (Americas). System updates don't come through Google Play (that's for apps), and while Google releases monthly patches and OS upgrades like Marshmallow, not all manufacturers release all patches or updates to their phones. Samsung is getting better and faster at it, though, but still not perfect. Anyway, if an update is available for your specific model and region, you can go into your Settings, About Phone, and check for updates there. Or use Samsung's Kies (their version of iTunes) to update via computer.