How challenging is it to change the mileage with a programmer? I found someone who has it, but says he can only copy the record from another counter.

I changed the mileage counter and I would like to set the correct mileage. The new counter has less mileage so I figured it would be straight forward, but I can't find the option to change the mileage anywhere. Am I missing something? I know I could change it on a 147. I'm connecting with a KKL cabl...