Update 6 from 5

Is it possible install cubase 6 update without the 5 version installed? If it’s not possible, how must I install it?

Yes. If 5 is installed, it will remain installed, as a separate program. If it isn’t, no matter. What you have bought is a full version of Cubase 6. The “upgrade” part of it is merely a financial discount for existing Cubase owners.