Yeah, I'd try using this button. If you don't have a manual, you can get PDF versions from the owners page at the Toyota website. The button might be in a different place on the pre-2012 models. I didn't check.
Holding trip will reset the displayed miles driven, mpg avg and mph avg back to 0. Find and tap the KM/H MPH button.
That's where it is on 2012-2015 cars too. US cars, at least--maybe that's why that manual page doesn't match.