First, add the QGIS repository to your system:
echo "deb [signed-by=/usr/share/keyrings/qgis-archive-keyring.gpg] https://qgis.org/ubuntu-ltr `lsb_release -c -s` main" | sudo tee /etc/apt/sources.list.d/qgis-ltr.list Finally, update and install (if the repo points to a newer LTR, you can pin the version): qgis 3.28 download
The official download page is: