According to Internet standard practice, these fields are expected to be unique for each webpage on your website. You can learn more about this here: https://moz.com/learn/seo/meta-description
It is possible that using the same meta description across all of your webpages will have negative effects on your website SEO.
However, if you are determined to have the same meta tag on every webpage of your website, any changes that you make to your header will appear on all pages of your website. You can access your header from the main menu by selecting "Settings" -> "Administrator" and then clicking on "Header" in the "Theme editor".