Add software repositories
Software is available from third-party sources, as well as from the default Ubuntu software repositories. If you want to install software from a third-party software repository, you must add it to Ubuntu's list of available repositories.
Only add software repositories from sources that you trust!
Third-party software repositories are not checked for security or reliability by Ubuntu members, and may contain software which is harmful to your computer.
To add a repository:
Click the Ubuntu Software icon in the Dock, or search for Software in the Activities search bar.
When Ubuntu Software launches, click Software & Updates
You will be asked to enter your password. Once you have done that, switch to the Other Software tab.
Click Add and enter the APT line for the repository. This should be available from the website of the repository, and should look similar to:
deb http://archive.ubuntu.com/ubuntu/ cosmic main
Click Add Source then close the Software & Updates window. Ubuntu Software will then check your software sources for new updates.
Most repositories provide a signing key to be able to verify downloaded packages. So in addition to the steps above, you should follow possible instructions on how to download and install the signing key (GPG key).
Activate the Canonical Partner repository
The Canonical Partner repository offers some proprietary applications that don't cost any money to use but are closed source. They include software like Adobe Flash Plugin. Software in this repository will appear in Ubuntu Software search results but won't be installable until this repository is enabled.
To enable the repository, follow the steps above to open the Other Software tab in Software & Updates. If you see the Canonical Partners repository in the list, make sure it is checked then close the Software & Updates window. If you don't see it, click Add and enter:
deb http://archive.canonical.com/ubuntu cosmic partner
Click Add Source then close the Software & Updates window. Wait a moment for Ubuntu Software to download the repository information.