Juicebox and Showkase are two different products (with different download links).
I've reactivated the download link for your Showkase-Standard purchase and you should receive an email with the link.
Once you have downloaded the current version of Showkase (v1.7.6), full instructions for upgrading can be found here.
(1) When upgrading your Showkase site, please be sure to make a complete backup of your existing site first, just in case anything should go wrong and you need to reinstate your original files at a later date. You should not encounter any problems upgrading Showkase but it would be wise to have a backup of your site should anything unexpected happen. Also, if you have made manual customizations to any Showkase source files, then please note that they may be overwritten when you upgrade and you'll need a backup in order to reinstate them afterwards.
(2) Please remember that Showkase-Standard comes bundled with Juicebox-Lite (the free version of Juicebox) so, after upgrading your Showkase site, you'll need to reinstate your Juicebox-Pro files by following the Installing Juicebox Pro instructions here.
Please let me know if I can be of any further assistance.