You can either download and install it manually, or install it automatically with the Plugin Manager, that you can find in menu Plugins=> Plugin Manager=> Show Plugin Manager
Open Plugin Manager
Open the Plugin Manager, look for the entry Python Script in the list, and select Install.
Select Python Script plugin
You might be prompted to update your Notepad++ or the Plugin Manager, please do so before proceeding.
Accept Plugin Manager update installationWait for Plugin Manager update installation
Once the Plugin Manager is updated, and after having restarted your Notepad++, try again installing the plugin.
Wait for Python Script plugin installation
You can now access in the Plugins=> Python Script menu.
Find new plugin Python Script under Plugins menu
The scripts accessible will be limited at first – you need to install required scripts in the folder (most likely under Program Files => Notepad++ => plugins => PythonScript => scripts)