Hello Karima,
No, it is not just an update of files, we did it a couple of weeks ago, and it
quite easy.
You first make one (or multiple) database backups, after that you uninstall the
server software, reinstall the v3 software, register the licenses, and of you
go.
Basically the server installation can be done in a couple of hours, and the
clients can stay on v2.1, or be upgraded later or at the same time,

Hope this helps

Fred