Savannah here from the Shopify Guru Team. :)
In order to change or add a CNAME to the domain you purchased via Shopify, you'll need your OpenSRS credentials. Shopify is a domain reseller and OpenSRS is the domain provider, so you'll need access to that account in order to make DNS setting changes. You can gain access to your OpenSRS account by getting in touch with us at firstname.lastname@example.org to request access.
Once you've had your login credentials forwarded to you, you can use this guide to access your CNAME records.
Let me know if there's anything I can clarify!
Savannah | Shopify Guru