There are two options to install a plugin into SonarQube :

Update Center

If you have access to the internet and you are connected with a SonarQube user having the Global Permission "Administer System", you can go to Administration > Marketplace.

Once download is complete, a "Restart" button will be available to restart your instance.

See Marketplace for more details on how to configure your SonarQube Server to connect to the internet.

Manual Installation

To manually install a commercial edition, see SonarSource Editions.

In the page dedicated to the plugin you want to install (ex: for Python : SonarPython), click on the "Download" link of the version compatible with your SonarQube version.  

Put the downloaded jar in $SONARQUBE_HOME/extensions/plugins, removing any previous versions of the same plugins.

Once done, you will need to restart your SonarQube Server.

License

If you installed a Commercial Edition, you will need to set the License Key in Administration > Configuration > License Manager before being able to use it.