Update, and upgrade - different procedure?

Hello, This may sound like a basic question - if it seems that way to you I apologize - but unlike an update (from 4 to 4.3 ) the upgrade of Dorico implies some payment.

So how do I update to D5 ?

Yeah, major updates are not free. Head to the Steinberg site and buy the upgrade there. There are numerous recent threads about how to navigate the installation process.