20.04 codename is Focal Fossa
You should go in the dell-foca folder :
http://dell.archive.canonical.com/updates/dists/focal-dell/Release
To download or upgrade packages, you should be using apt and have your /etc/apt/sources.list properly configured.
https://www.dell.com/community/Linux-General/Ubuntu-Desktop-20-04-LTS/td-p/7562171
http://dell.archive.canonical.com/updates/dists/
should be