As your domain name has already been purchased and registered, it is not possible to change the name. This is not unique to Yola, but it has to do with the way domains are registered on the internet - they are always non-editable once registered.
What you can do is buy a new domain to use in conjunction with your original domain. This can increase traffic to your website and is also excellent for SEO.
To prevent content duplication, we add a 301 redirect to one of your domains. You can read more in this tutorial about 301 redirects.
Click on this tutorial on how to buy a new domain for your site.
Once this is done you can set your new domain as your primary domain, so that your other domain redirects to it, by following steps in this tutorial: Publishing your domain and email.