AFAIR you need to set resolution to 12 bit, in 8-bit it won't work. Then you pull up Vref (UNBUFFERED PIN!) to desired voltage (No more than Vcc though!) to extend the voltage range. (I don't know if you should or shouldn't do this through a resistor though)

Method 2: Just split the input voltage. If your input voltage changes from 0 to 25V and your readout is 0 to 2.5V (default) then just put 900K resistor between readout point and ADC and 100K between ADC and ground, thus you will always get 1/10 of the readout voltage on the ADC.

o-- 0..25V
|
[]900K
|
o--ADC 0-2.5V
|
[]100K
|
o--GND