You can access the DNS records for your Shopify purchased domain by going to Online Store/Domains. From there, click on the domain, then "Manage DNS" settings. Step 2 would indicate that you have a second store setup. If that is the case, then you can add the subdomain to the new store by clicking "Add existing domain" in Online Store/Domains - If you get an error that states the domain is connected to another store, then you'd need to disconnect the subdomain from your first store to allow it to be added to your second store.
Let me know if you run into any issues.
Trevor | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog