You can do the update by re-importing the new pbix file.
There's an argument nameconfilict for PostImportWithFileAsync, and as per this github sample, the argument value "Overwrite" can be used to overwrite the already existing dataset. However "Overwrite" seems a invalid status code based on my testing and I haven't found the correct status code yet so far.
As a workaround, you might to use the PowerBI-Cli to overwrite, check Updating existing workspaces with a new pbix file #22.