You can install wordpress plugin in several ways . You can follow any of these steps :

Before installing , You need to have install & activate WooCommerce plugin.

Automatic Installation

1. Navigate to the WordPress Dashboard > Plugins > Add New
2. Search for Pre Order Addon for WooCommerce.
3. Click on Install and then Activate button.

Manual Installation

1. Download plugin from wp.org and unzip it.
2. Upload the entire folder to the /wp-content/plugins/ directory.
3. Visit Plugins.
4. Activate the Pre-Order plugin.