hi is it possible to underclock a atmega328pu chip to 16mhz?
Yes. That is what the Arduino Uno does. You can underclock the AVRs all the way down to zero Hz, as far as I know. (A stock Atmega328p-pu runs at 1MHz, using an internal 8MHz RC clock with an additional divide-by-eight.)
so i just put an16mhz crystal there?
Yup - just take off the 20mhz crystal and put on a 16.
You'll have to reburn bootloader afterwards if it has a bootloader built for 20mhz on it (else the serial baud rate will be wrong and you won't be able to upload to it).
thank you for the reply