How to rename the Azure WebApp URL
It's not possible to rename web apps.
And you don't need to delete the whole resource group. Just delete the one web app and recreate it with the correct name.
You can clone the web app , and in the process give the new url.Remove the earlier web app and you are good to go
You can just add the hostname to the resource hosting the web application.
For example from portal.azure.com go to "App Services", choose your service and from the blade search for "domain", then click "Custom domains", then add "hostname" that will ask your DNS, validate and give you the DNS records you need to add, see screenshot below:
NOTE: this does not actually rename the resource, the old one will still be a valid alias for the the resource along with the domain name registered in this note.