how to update Witcher 3 on GoG

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I bought Witcher 3 in a local store at the time, so I installed it through discs. I've noticed the game hasn't updated since then though. In-game it says I'm still at version 1.02, though the changelog of the GoG Galaxy program talks about version 1.08 being implemented already.
Does anybody know how I can update Witcher 3?
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How To Update The Witcher 3 Wild Hunt GOG (NEW Patch 1.31 !!!) Standard Version
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