Boot Camp 6 update won't install on Windows 10
It has been a while sine I tried this so my apologies if it is a little vague... But the actual Windows drivers are inside the BootCamp package. In a zip file if memory serves.

Right-click on the bootcamp app and look for the drivers zip file (should be obvious which one it is.) and copy it out.

Put it in a place that your Win 10 installation can see it, boot into windows, copy the file over, open it up and install the drivers. One at a time if necessary in Windows.