Upgrade machine to mojave from high sierra HFS

Hello,

We have a parallels machine : mac os 10.13.2 configures to HFS
We want to upgrade the machine to mojave. During the installation there is an error saying the drive hsould be APFS...
Also, tried to download a new 10.13 machine - but we could not find a download link (we can create a new machine image with APFS)...

Can you pls intrstuct us on where to download a 10.13 image version ? or a secondary option - upgrade our system to APFS and then to mojave (a manual will help us on that subject)

Thank you for your help !