You can simply Install and Active this Plugin in 2 different ways after Logging in to WordPress Dashboard:
- Click on the Plugins shown in your Dashboard
- Then go to Add New option
- Search WP Post Author in the search bar
- Then, Install and Activate it
OR
- After Add New option by clicking Plugins in the Dashboard
- Click on the Upload Plugin button shown top of the left side
- Choose the zip file that you have downloaded from WP Post Author
- Then, Activate the Plugin