Which method is NOT a way to install an extension in Magento?

The method identified as not a way to install an extension in Magento is using third-party software. This approach is not a standard or official method supported by Magento for installing extensions. Magento primarily encourages using its built-in tools and interfaces for extension management to ensure compatibility, integrity, and security of the platform.

Magento Connect Installer allows users to search for, install, and manage extensions directly from the Magento marketplace, which is a supported and straightforward process. Copying installation files to the server is also a valid method, where users manually upload the necessary extension files and follow the installation procedures. Additionally, the option to install extensions directly through the backend admin interface is a user-friendly approach that Magento has made available, allowing for easier management of extensions without needing deep technical knowledge.

Utilizing third-party software for installation can introduce risks such as compatibility issues and may not comply with Magento's best practices, which is why it is not considered a valid installation method.