Use a resistive divider to attenuate the incoming voltage so it doesn't exceed the reference voltage - you don't want to exceed the reference voltage, and (even more importantly) you don't want to exceed Vdd.
please use a resistor divider and care should be taken to avoid problems due
to the tolarence of the resistor. you may advised to use a trimpot in the resistor divider to overcome the problem. if it is a precision application use firmware calibration.