I'm using Lightroom 6.00 (retail version not cc) and when I try to start the update to 6.4 (update function in the help menu) it tells me everything is up to date :-(
Assuming your operating system can handle Lightroom 6.4 (see here), you can download the latest installers from Keeping Lightroom Up-to-Date
Try a direct download
Go to LR6.4 & LR CC 2015.4 Updates
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