sudo. Modify as required.
sudo wget -O /usr/share/keyrings/nickh-archive-keyring.gpg https://www.ne.jp/asahi/nickh/debian/nickh-archive-keyring.gpg
sudo wget -O /etc/apt/sources.list.d/nickh.sources https://www.ne.jp/asahi/nickh/debian/nickh.sources
apt show river).
If apt complains about an invalid signature. Eg
Err:10 https://www.ne.jp/asahi/nickh/debian trixie-backports/ Release.gpg
The following signatures were invalid: EXPKEYSIG 683B67D7BC99A9B7
you may have an old expired repository key. Please repeat the "Install key" procedure above.
If you install the nickh-archive-keyring package, you should not need to do this.