Cubase 7.5 will install independently from 6. If you already have 6 installed, you don’t need to do a thing. You might want to leave 6 installed for familiarity while learning the new features of 7.5.
I would install from the complete 7.5 ISO images in Fabio’s link. First, install the latest eLCC and get your license up to date.
After that, you should be good to go.