Do you mean the voltage range?
have a look in the programmers guide at the ps4000SetChannel function
This is where you specify the voltage range you are using.http://www.picotech.com/document/pdf/ps4000pg-en-5.pdf
Then to get the mv value back from the adc value returned from the scope, use the equation.....
mV = (ACD count x mV range) / max value (note: max value = 32764)
so on the +/- 5V range, an adc count of 12950 would give a 1976 mv value
(12950 x 5000) / 32764