Hello, Richard. You did not let us know about the DC-3, is it working now?
You need to download 2 files from MTH: the 5.0 Loader and the 6.1 DCS file (you can upgrade directly to 6.1 from whatever version you have).
It sounds as if you have already extracted the 5.0 Loader program. If so, you will need to extract the 6.1 file into the same place. This isn't required but, it easier to find the programs. The extract will create 2 files: tiu6p10.dcst and remote6p10.dcsr.
Now you should be ready to upgrade.
I see GRJ posted a few moments ago. We are making an assumption that you have done this before and have the wires you need, as John said you need that stereo wire and you will also need the cable that connects the handpiece to the base of a hardwired telephone to connect the remote to the TIU for upgrade.