Which one? Your user account password or the root password?
You can change your own password at any time, simply open a terminal window and type
You'll be prompted:
Changing password for user_name
Old password: <type it>
Enter the new password (minimum of 5 characters)
Please use a combination of upper and lower case letters and numbers.
New password: <type it>
Re-enter new password: <type it>
passwd: password changed.
Note, where you see <type it> you won't actually see anything as you type.
To change the root password it's pretty much the same but you need to log in as root or use sudo
or use su -
and you need to know the old root password.
Hope this helps some.