Download the new zip file
Go to the website where you bought Fluent.
2-Find the order page in your profil menu.
/!\ If you bought without account creation, reuse the mail sent by Blendermarket, and click again on the link in the mail.
2-Find Fluent in your library.
Uninstall the previous version
Acess to the add-on list.
Search and desactivate Fluent. For an efficient update, my advice would be to remove Fluent.
Install the new version
Restart Blender and use the basic install process.