Add (at least temporarily) a download repository. This requires registering trusted key. (More information). Run the following shell commands to trust the repository:

wget -nv https://download.owncloud.com/desktop/ownCloud/stable/latest/linux/Ubuntu_23.10/Release.key -O - | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/owncloud-client.gpg > /dev/null




Run the following shell commands to add the repository and install from there:

sudo apt update
sudo apt install apt-transport-https
echo 'deb https://download.owncloud.com/desktop/ownCloud/stable/latest/linux/Ubuntu_23.10/ /' | sudo tee -a /etc/apt/sources.list.d/owncloud-client.list
sudo apt update
sudo apt install owncloud-client