- You can't, unless you bought C9 within a very short space of time before C2018's release, in which case I suggest contacting customer support about it; and
- Are you sure you even want to?
For me, C2018 has brought nothing but annoying niggles. None of the new features are important to me, and let's be honest, there aren't that many of them in any case. In my humble C9 is considerably more stable and I think that after I finish this current round of projects I'm going to go back to it. If C2018 has new features that you need, they're probably worth paying for if you bought C9 a while ago. Otherwise, I'm not sure you're missing much.