Power up pasword. just purchased a compaq n400c can u tell me how i change the power up password

Question

just purchased a compaq n400c can u tell me how i change the power up password as i dont have a manual , i do know the current password , just want to change it , thanksi removed the battery on mine, i have a compaq presario nc6220, and i cannot bypass, even when i press f8, it wont let me in, what do i do????