Hello,

No, of course you dont have! All updates are free.

Just open the asset store editor window in Unity and you will see all your bought assets. From there there is an update button. The window even lets you know that a newer version is alive.
So everything is done through Unity and menu “Window/Asset Store” editor window.

PS: Before update it’s always better to delete previous installations (meaning NodeCanvas folder) and re-import anew.
This is a good practise for all assets. Not just NC.
Just keep a backup for safety if you have to.

🙂