Daniel at Magnum bikes promptly responded to my question with a solution that worked. Here's his answer:
Hi Bill,
Thank you for contacting Magnum Bikes. It is a little tricky resetting the trip distance. What you want to do is:
1. Hold the SET button for 3 seconds
2. In the setting menu that...
I've had my Magnum Ui5 for a month or so now and am really pleased with it.
However there are a couple of display issues that I haven't been able to resolve-
Specifically, I'd like to be able to reset the trip distance and trip time to zero while retaining the overall Odometer mileage. On page 7...