As Toyota mentioned in one post via Prodigyplace: about changing mph to kph, it can be changed however. Friend of mine is bringing priuses from US to Europe and one can change metric system either via steering wheel buttons or via radio settings (genera/language and then pick mph or kph). Hope this help (i got photos if anyone in need)
The old/quaint method for changing units on third gen: This is where Toyota is cheaping out, relegating ever more basic functions to the convoluted touch-screen menu, pretty much requiring you to pull over and park, pull out the owner's manual.
Not on US cars manufactured before March 2016, though. That is what started that other long thread. Notice the owners manual posted online by Toyota is just for the newer ones. My 2017 Prius, manufactured in September 2016, can change fine.
Please post in the Prius v part of the forum. Your car is not a Gen 4 Prius. It is closer to a Gen 3. European firmware likely will not let you change to miles.