Download the TU and put it on your 360. While you could use FTP to do so, it might be easier for you to just use the program, Horizon.
Google and download horizon, open the program and open a package manager within Horizon. Use the package manager to open up the TU. Format a USB on your 360 on the retail dash and then plug the USB into your computer while Horizon is open. Move the TU in Horizon over to the USB via the options within Horizon's package manager. From there, put the USB back into your 360, go to the retail dash, go to storage devices, find the USB, and then move to the TU over to your HDD. Your 360 will put it in the correct filepath for you automatically this way.
As a side note, the fixed version of FSD rev775 can download TU's.