If you'd like to change your password, just log in to TVNZ and head to your Account page. If you’re using a PC, click your name in the top right of the window to find Account options. Click Account and then click change password. You'll then need to enter your new password twice, along with your old password and select the update button.
In the app, the Settings option in the menu will direct you to head to www.tvnz.co.nz in a browser on your device to complete the steps above.