Use the Admin Panel/Download Plugins.

You can also download Plugins (and Themes) manually and install them thusly:

Extract the contents of the ZIP archive into your /themes or /addons directory.

Afterwards they are ready to install via either Admin Toolbox->Plugins->Manage->Available(n)

or for Themes Admin Toolbox->Appearance->Available Themes

Uninstalling Plugins

A) Admin Toolbox -> Plugins -> Manage. Open the respective 'Options' section of your installed plugin(s) and click 'Uninstall', or
B) Admin Toolbox -> Plugins -> [Plugin Name]. Here the Options are a separate panel and 'Uninstall' is already visible.