On the touch screen go to Configuration, top right. There you will find some sub menu's such as audio, brightness etc one of these is Configuration again.
Here you can change Celsius to Fahrenheit and I also believe mph to kph but not having done it myself I am not 100% sure, but it's worth a look.
Latest Cactus with Navigation:
On Right-Hand Side, Top of Screen Press "2 Cogs Diagram" (Left of Clock)
Next Screen, Bottom Left Press "OPTIONS"
Next Screen, Top Left Press "System Settings"
"Distance and Fuel Consumption"
On the Right-Hand Side, Toggle between "mpg-ml" and "km/l-km" Press "OK" (Green)
Speed/Distance will change to Miles or Kilometers, depending on selection.