Update a Subscription through REST

Hi,

We're trying to update an existing Subscription through REST. I am able to GET a Subscription successfully but I am unable to update the same Subscription with a PUT. I am looking to update the BillBegin and BillThrough fields specifically. I tried to use: the endpoint - ../Subscription/{PartyId} for which I got a 404 and the endpoint - ../Subscription?{PartyId}&{ProductCode} for which I got a 405.
I believe these are editable based on the article here - https://developer.imis.com/docs/adding-modifying-and-paying-for-a-subscription. But we're missing information on how to modify these fields.
What does the API need for a valid call? Please let me know.

Thanks,
Ram Devarapu