You can login there with the email you used when buying the plugin and the password that was emailed to you at that time. Use the lost password page if you can’t find it anymore.
If you bought the plugin before February 2014 you’ll have to register an account first with the email address you used to buy the plugin.
Once logged in you can click through to your purchase details and find your download at the bottom of the page.
Once you have the latest .zip file you basically have to follow the Premium installation instructions again, with one added step: deactivate and delete the current Premium plugin first. Otherwise WordPress will complain that the plugin folder already exists. Deleting the current version will not remove any of your recipes or settings.