Read this
tutorial and your pics datasheet. You could measure more than 5 volts using a voltage divder.
For example if the maximum voltage is 10V, you could divide it by 2 and drop it to 5V. 5V is the maximum voltage a pic can handle so by measuring 5 voltage you can multiply it in the software part knowing that is actually 10Volts. You may use two voltage dividers if the Vmax is 20 and divide it by 4 on the hardware part and multiply by 4 on the software part. I hope it helps!