You can change your store name anytime. In your Store Admin Panel, navigate to Settings > Business Info.
You can change your store URL anytime after you have opened your store. In your Store Admin Panel, navigate to the Storefront tab and then to the Overview section. Just remember, the old URL will no longer work, effective immediately.
If you're store is currently "Closed" turn it to "Open" by going to Settings > Store Settings and select to open your store. This will give you access to change your store URL.