setting a vCACVmProperties custom property in a vCO workflow..

Apologies if this has been asked before, ( tried searching and didn't get a hit), also am new to vCAC/vCO :-)

I've no problem getting the current value of a property eg:

var osFamily = vCACVmProperties.get("Custom.OSFamily"),

However, based on some customization that goes on within the vCO workflow,I need to update/change the value stored within vCAC, so wish to change the vCACVmProperties  'property to a new value, and I can't seem to find out how ?

     Thanks in advance,

                    john.