I figured it out.
First, the URL should be: http://localhost/reports/api/v2.0/Subscriptions(f6b3ff9a-9844-4d11-a61b-d2bfc02c84f4)
Second, the body should be:
{
"Id": "96ad5f4e-da19-41a2-8fed-9929105b37a8",
"ParameterValues": [
{
"Name": "pRegion",
"Value": "Atlantic",
"IsValueFieldReference": true
}
]
}