After you've logged in on your OnSign TV account, click on your name on the right upper corner, then click Settings. On the displayed page, click on FTP Settings on the left menu. Delete the written password and enter the new one. Remember to click Save Settings button before leaving the page.
Check out on this tutorial how to use FTP client FileZilla.
See "How do I change my password?" if you want to change your OnSign TV account main password.