To change your password for your web site, you just need to login, go to "my account", click on the edit tab and put in your new password. If you've forgotten your existing password and so can't login, use the "Request new password" feature.
To change your email password, you need to login to the system shell. Find instructions on this here. You just need to install PuTTY and open a session, then login. Once this is done, type the following:
username@acrewoods:~$ passwd
Put in your current password, then type out your new password twice. If you've forgotten your current password, contact tom and he will generate a new one for you, which you can then change.