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If you need to update then go to the website and in this website you want to click on Arnold and then go over to downloads. Go up to the main menus and choose Arnold About and you'll see the version listed here. You can check the current version of your Arnold plugin from the Maya menus.
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And so it seems there's an update every few weeks, and it's kind of on you to be on top of this to pay attention to the updates and download them as needed from the website. In fact, the Arnold plugin and in fact the Arnold core are updated maybe six or ten times a year. But you'll find that the Arnold plugin known as M2A is updated much more frequently than Maya. The Maya application gets a major version update about once a year and it gets a minor update or point release a couple of times a year depending upon whether they are a lot of bugs to fix or new features to be released.
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