Turn off the guest
Create a snapshot, its not required but is good practise.
Remove the guest from the inventory (Right-click -> remove from inventory)
Browse the datastore your guest is stored one and download the guest’s [name].vmx file.
Open the [name].vmx file on your workstation and change the “virtualHW.version = “10″ line to virtualHW.version = “9″
Use the datastore browser to upload the file to the original location
Add the guest back to your inventory by right clicking the vmx file and selecting “Add to inventory”
Start the guest