(parenv) jetson@jetson:~$ sudo apt update
Hit:2 http://ports.ubuntu.com/ubuntu-ports focal InRelease
Hit:3 http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease
Hit:4 http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease
Hit:5 http://ports.ubuntu.com/ubuntu-ports focal-security InRelease
Hit:6 https://repo.download.nvidia.com/jetson/common r35.4 InRelease
Hit:7 https://repo.download.nvidia.com/jetson/t234 r35.4 InRelease
Ign:1 https://repo.download.nvidia.com/jetson/x86_64/focal 36.2 InRelease
Err:8 https://repo.download.nvidia.com/jetson/x86_64/focal 36.2 Release
404 Not Found [IP: 23.196.14.82 443]
Reading package lists... Done
E: The repository 'http://repo.download.nvidia.com/jetson/x86_64/focal 36.2 Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Something is seriously wrong with package configuration if anything x86_64 shows up in apt. If someone manually added something to force a package, then this has harmed the package system. Nothing x86_64 will work on a Jetson. I hate to say it, but if this is the case, then I would suggest flashing the Jetson and starting over. You could attempt to revert the x86_64 packages, and then remove that repository, but it is hard to say how that will work out.
The page you are referring to only works for tools on your host PC, but not Jetson devices.
Upgrading from 35.x to 36.2 will not work, at least for now, no matter what repositories you are working on.
Please re-flash the device manually.