There are two different indications of the guest OS.
One you can do when you create the VM with the New-VM cmdlet and the GuestId parameter.
And you can change that setting with the Set-VM cmdlet, again with the GuestId parameter.
The values you can specify are defined in the VirtualMachineGuestOsIdentifier enumerator.
But this setting is not binding, in fact you could pass the value windows9Guest, which would mean Windows 10, but you could still install for example a Linux OS on that VM.
The second one is the name of the guest OS, as determined by the VMware Tools (so you need to have VMware Tools installed to have this working).
This one you can not change.