You can use the <dpkg> package manager to install downloaded .deb packages on Ubuntu and other debian-based systems.

To install a package pass in the -i flag:

sudo dpkg -i <package.deb>

You can list installed packages with the -l and an optional pattern:

dpkg -l <optional pattern>

This list can be quite long, so it might be helpful to redirect the output into a file.

dpkg -l > dpkg.txt

Use -r to remove a package.

sudo dpkg -r <package>

-r uninstalls the package, but it leaves any configuration files in place so if you ever install that package again it'll already be initialized.

To remove all associated files use -P to purge all the data for the desired package.

sudo dpkg -P <package>