The video below shows the process of update page meta description (click the button on the bottom right of the video to expand the view). You can also follow the screen shots and steps below this.
Your Meta Tag Description is description of what your website is about. The data is held in your websites background code, and is only visible to search engine spiders. It should be constructed as one paragraph or sentence to a maximum of 255 characters. Your description should be in plain english using as many of your Meta Tag keywords as possible. The description must make sense to human readers as this description can be displayed by Search Engines on their results pages.