Custom domains are your own domains in the format www.example.com, while subdomains are like https://example.myshopmatic.com with the suffix myshopmatic.com present in the URL
Update your primary domain from a custom domain to a subdomain by going to https://myshopmatic.com/manage/domain_name [Left Menu -> Setup -> Domain] and selecting the subdomain from the dropdown menu and clicking on Save.
It is advisable to check the subdomain in a new browser or incognito mode to confirm that the subdomain has been updated correctly.
Do remember that you will need to use this updated subdomain across all mediums. This means adjusting marketing campaigns, social profiles, blogs, posts, etc. to include subdomains wherever custom domains were used previously. All the buyer traffic should be directed to the subdomain.