To change VNC Password on Linux You can either connect to the machine remotely over an ssh program like putty or you can do this directly from the terminal.
Open up a command prompt for the user that you have vnc server running as.
You will then be prompted to enter a new password twice. Choose your new vnc server password and then restart the vncserver service using the following command.
service vncserver restart
The above commands work when run as the root user on Centos but should work similarly on most other variation of Linux.