Have you a connecting cable, Kate? For example, this one?
I'd tried using a connertor cable between my old serial cable and a USB cable, but no joy, so ended up paying full whack for it. It is a lot for a cable
but it did the job 
Once connected, you can import marks, routes and tracks from the GPS unit. And you can export them from the mapping software too.
If you export your "overlay" files regularly, that acts as your backup in case of problems.