User “nvidia” should be pass “nvidia”, user “ubuntu” should be pass “ubuntu”. If either of those can be logged in to, then you can use sudo to change the password of the other.
If you’ve customized passwords and can no longer remember for one of those two accounts, then your options are limited to some fairly time-consuming methods to change it. You could flash your board and start over fresh. Alternatively, you could clone your Jetson, place the password files on top of the loopback mounted clone using the sample rootfs (which would reset passwords), and then flash the Jetson using the clone instead of a new image (you’d preserve everything on your Jetson, but you’d still be essentially flashing…this isn’t fast to clone and flash).
Are you able to log in to either user ubuntu or nvidia? Do you need info on cloning?