I'd recommend finding a good plugin. Just search the Wordpress Plugins directory for such a plugin and activate it in your theme. A quick search turned up this one: http://wordpress.org/plugins/add-meta-tags/, but feel free to feel out others. If you edit the header.php files directly without using PHP, you will have the same header information for every page of your site, which would not be good... A plugin would help simplify the task of creating custom tags for each page/post. All care.
If you're new to Wordpress All in One SEO Pack is a good option otherwise I'd go with the more powerful and customizable Yoast SEO plugin. Both allow for you to set various meta tags, titles, descriptions, etc.
Are you talking about wordpress cms or blog? if cms you can update your meta data by adding plugin called all in one seo, seo tool etc. There are so many that you can use.
I have used All-in-one SEO plugin for most of my clients project. But you can also try Yoast SEO plugin . Besides if you dont wanted to use any third party plugins & if you know PHP/HTML , you can directly embed you meta tag info on the header pages.
If you are looking to use some sort of software there is no better then All In One SEO Pack like mentioned above, however if you are not looking to use a plugin such as this try this: http://wordpress.org/plugins/add-meta-tags/