If you already own a license for Cubase 5, getting the 5.5.3 update is straightforward and recommended. This was the final "maintenance" update for the version 5 cycle, fixing numerous bugs and improving 64-bit performance.
But if you are a new producer reading this: buy Cubase 14 (or 15). You will thank yourself when you discover features like Sample Track and the built-in Dolby Atmos renderer. Cubase 5.5.3 Download