code: public Observable<OperationStatusResponseInner> startOSUpgradeAsync(String resourceGroupName, String vmScaleSetName) {
          return startOSUpgradeWithServiceResponseAsync(resourceGroupName, vmScaleSetName).map(new Func1<ServiceResponse<OperationStatusResponseInner>, OperationStatusResponseInner>() {
              @Override
              public OperationStatusResponseInner call(ServiceResponse<OperationStatusResponseInner> response) {
                  return response.body();
              }
          });
      }
docs: Starts a rolling upgrade to move all virtual machine scale set instances to the latest available Platform Image OS version. Instances which are already running the latest available OS version are not affected.
  
  @param resourceGroupName The name of the resource group.
  @param vmScaleSetName The name of the VM scale set.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable for the request