To download QGIS 3.26 "Buenos Aires" , you can access it via the official QGIS Archive OSGeo Download Server . While current versions are available on the main QGIS website
QGIS allows multiple versions to run side-by-side. You can keep QGIS 3.26 installed while downloading QGIS 3.34 LTS to another directory. qgis 3.26 download