Below are instructions on using Metafields in Shopify which is used to edit custom fields.
1. Install the metafield app by going to this link: https://apps.shopify.com/metafields-editor-2 (You will need to login using your Shopify logins first before installing)
2. When installing you will get taken to a page like below asking you to confirm the installation by clicking the “Install app” button
3. Once installed you will get a window like below with a list of resources. Select the relevant resource which you want to edit (Usually this is products or collections)
4. Once you select a resource from the list you will get a list where you should navigate to the specific product/collection you want to edit or add the metafield.
5. Once you are here you will have the options like below to editing your metafield. This consists of a key, namespace, value type and the text which appears on the metafield. (These should be sent to you in a seperate document/email so make sure you keep these somewhere easily accessible)