This is Max from the Shopify Team.
Each account that you sign up for with Shopify comes with a custom .myshopify.com URL. This name, cannot be changed. You can change many things about the store name, account details, etc., but, where we store all of those details is linked to the specific .myshopify.com URL.
If you've been working on a trial and your business plan or name has evolved and you want to adjust the URL, you have two options;
Please let us know if you need any more clarification or help with next steps!