My name is Rae and I work for Shopify. I'm happy to lend a hand with this!
Can you please clarify which part of the copyright information you're trying to change?
For instance, if you're trying to edit your store name, you can do this by following these steps:
For reference, the field you should edit is here:
Once you've completed this, go ahead and visit your live site to check to see if the store name is displaying correctly within the copyright details in your footer. If your previous store name is showing up instead, make sure to clear your cache to help update the information on your end.
Let me know how this goes, or if you're looking to edit different information instead, and I can continue helping out!