This is Max from the Shopify Team.
Each .myshopify.com URL is created to track an account and its details. It cannot be changed after a free trial has been established. A new .myshopify.com URL can be created with a fresh sign up if you did want to adjust the name (this would require you to start from scratch).
If that's not what you were after, are you looking for some help to close your Shopify account? If so, this doc should help you out with that!
Please let us know some more specifics about your question if these options did not answer things for you!
You can do this by closing your shopify account with shopify.
Go to the admin page - settings - account and close your store.
If you want to keep your shopify store, the best way is to just redirect the store to the domain of your choice.
In this case, your shopify URL will still still remain with your account.