You can directly edit those files, the lines should look like in the link to the Brave issue above. From the terminal you can type something like sudo nano /etc/apt/sources.list.d/<name_of_the_brave_source_file>. All you need to add is "arch=amd64" inside the square brackets along with the "signed-by" setting:
deb [arch=amd64 signed-by=/usr/share/keyrings/brave-browser-nightly-archive-keyring.gpg] https://brave-browser-apt-nightly.s3.brave.com/ stable main
If you are using nano you can press Ctrl+O to save the changes, and then Ctrl+X to exit.
Note that you need elevated permissions to edit those files, which is why you need to prefix that command with sudo. However you can also use more user-friendly text editors, so you might want to check this thread first to learn how to do that using root privileges.
If you run apt list --installed | grep brave do you have any matches for brave?