code: public OperationStatusResponseInner beginStartOSUpgrade(String resourceGroupName, String vmScaleSetName) {
          return beginStartOSUpgradeWithServiceResponseAsync(resourceGroupName, vmScaleSetName).toBlocking().single().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
  @throws CloudException thrown if the request is rejected by server
  @throws RuntimeException all other wrapped checked exceptions if the request fails to be sent
  @return the OperationStatusResponseInner object if successful.