I bought the Witcher 3 and have no idea how to update it to the lastest patch. I'm still stuck in version 1.02
Go into your Library or click "Account" scroll (if need be) to The Witcher 3 on your shelf. Click on it and it should have a list of patches, download all necessary (in your case 1.03 and 1.04). Once they are downloaded go to where you saved them, simply double click on those the patches and install them. Once you do that you'll be off and running with the latest patched version. Though some have had issues with 1.04 so keep in mind if you have any issues. That being said it runs fairly well for me minus a few graphical hiccups. I hope this is of help to you, and good good luck. :)
edit: I'm assuming it's the PC version, otherwise please disregard me, lol. Also, that's if you either bought it on GOG, or have redeemed the code with the physical version. Otherwise I'm not rightly sure where to patch the retail version of the game.