how to upgrade Tabs to premium version without loosing data?

Hello,

Currently using free version of Tabs plugin for wordpress with several tabs made. Just purchased Premium version to be able to use shortcodes but when I tried to install it, wordpress says "Destination folder already exists. Plugin installation failed."

readme.txt file included in the plugin zip does not mention upgrade process.

Any help on how to upgrade to Premium keeping the tabs made on free version is highly appreciated.

Thanks in advanced.