Install and use Shopify Metafield Editor
To install the app, just visit the app listing page on Shopify app store by clicking here.
After installing the app you will find yourself in your shop`s admin.
The usage part is also very simple. On the first page you will see all the resources available for editing custom fields.
After choosing one, let`s say products, the next page will list all of your store`s products.
Choose the desired product and on the final page you will see a form where you will type in the metafield and on the right side there will be a list of metafields (if any).