If your site is published to a free subdomain, you can change it by following the steps outlined here.
To change or modify the domain name purchased from Yola, please read "I bought my domain, but I want a different one" under the Frequently Asked Questions section in this tutorial: Buying your own domain name from Yola.
You may also find this useful:
How do I buy my own domain name from Yola?
Does Yola offer country specific domains?
Adding domain purchased from Yola
Transferring your domain to Yola