To install the extension/plugin downloaded by you:

1. Go to the “Wp-admin” panel.
2. After that, go to the “PLUGINS” as mentioned in the screenshot:
   
3. In this plugin section, you have to hit the “ADD NEW PLUGIN” as mentioned in the screenshot:
   
4. On clicking add new, you have to “UPLOAD PLUGIN” as mentioned in the screenshot: 
   
5. After hitting the upload plugin, it will appear a page were you will “BROWSE” the zip file of the plugin from the folder which you have saved in your system after downloading as mentioned in the screenshot:
   
6. After Uploading, hit the “INSTALL NOW” tab as mentioned in the above screenshot to install the plugin.