.deb files are the installation files for software in the debian / ubuntu flavours of linux.
To install these, use the “apt” command. Don’t use “aptitude” or “apt-get”.
For this example, I downloaded the text editor “Kate” from https://ubuntu.pkgs.org/20.04/ubuntu-universe-amd64/kate_19.12.3-0ubuntu1_amd64.deb.html using wget.
sudo apt install kate
Once installed, you can run an update to make sure you have the latest version by running:
sudo apt update
and
sudo apt upgrade
Done!