You can get the free version in the WordPress repository at https://wordpress.org/plugins/social-booster/
And you get both the free or the premium version here: https://rextheme.com/social-media-auto-publish-and-schedule-plugin/
Go to your WordPress dashboard and select Plugin > Add New. Search for the plugin with Social Booster and you will find our plugin as shown in the image below:
Click on Install and then Activate.
If you download from RexTheme website, go to Add New Plugin > Add New. On top, you will get the option to Upload Plugin. Simply upload the plugin and activate it.
Once the plugin is installed and activated, you will find the plugin added on your dashboard: