I have downloaded it and installed but how to use it?

After installation, you just have to activate the plugin, from the plugin section in WP. After that you should see an extra column in your All Posts section with the counts per post. The same with All Pages. An extra Box with Post View Counter graphs should be visible on your WP admin dashboard. In Settings, there should be an extra Post View Counter section. When you edit a post, you can change the number of counts for that post. Same with Pages.