All your passwords can be changed through the JTAN members page
Log into that page. Click on the config icon for each service you want to change. The config icon looks like a "wrench". Passwords are set by clicking the golden key icon on the config page. You can change ftp/shell/pop passwords this way.
If you want to reset the overall members password, use the "Service: Passwords" link on the lower right of the main members page and explain that you want a new members password. Be very careful when you do this. If you lose the members password, you lose the account.
You should review the contact information on the upper left of the members page and correct the name/address/email/phone contact information.
Password changes can take a while, so be ready to use your old password till the new one takes effect.