Depending on your hardware you may need to download a prepackaged instance of ESXi from the manufactures website.
For example I did this process on a Dell Poweredge 610. The only available download was version 6.0 so I installed that version.
I was attempting to convert my PLEX server from Hyper-V to ESXi however I kept running into errors and come to find out I need version 6.5 to support vmdk files larger than 2TBs
So I then downloaded ESXi 6.5 for Dell Poweredge 620. Even though the 610 is not supported the upgrade worked.
Write the ISO image onto a flash drive (I used rufus) then boot and run through the install and when selecting the drive ESXi 6.0 is installed on it said this:
Be sure Upgrade ESXi, preserve VMFS datastore is selected and press enter
I did get errors when creating a VM though and everything pointed to my converted vhdx to vdmk file so I had to rebuild the server information but the upgrade worked with no errors.
My 2TB converted vmdk error for reference:
Error: File system specific implementation of LookupAndOpen[file] failed.