Version 5 is the previous version and only security updates and bug fixes are done on that version. It is only kept because some customers don't want to upgrade (requires buying an upgrade) to the latest version where all the new features are implemented.
If you had bought a licence for V5, then you can update to v5.8 for free, whereas upgrading to the latest v6 would require you to pay a minimal upgrade fee.
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If you want to upgrade, please download the test script from the Free Download page and verify the trial version will work on your server. There are many additional features in Version 6. Just take a look at the demonstration page and try out the new Event Manger to see some of the new features.