It depends on how you want to do it. If you want to swap out the old version for the new one then you can do it by deleting the old theme (back up/save first) and then replace it with the new version.
If you want to carry over some of the customizations from the old version, then I would manually add those in when you upgrade. I usually create another subdomain as a demo to test everything before swapping everything out.
You can download the latest version from your account page here – https://appfinite.com/my-account or if you purchased from StudioPress you can find it here – https://my.studiopress.com/
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