Like any other programming language if you need to update multiple fields, you pass multiple parameters to the function. The URL should be equivalent to namespace/class/method while the parameters are just that - parameters you can pass in JSON format.
You're putting the burden of learning implementation details of your system onto your customers. That won't be comfortable for either them or you. Should you need to deprecate a method you'll never be able to remove it. So be prepared to write a lot of { "Error": "ErrorCodeThatOnlyMakesSenseInTheContextOfThisAPI", "Description": "DEPRECATED" }
