The address of your site can be changed at any time via the Settings tab in the editor. Open up the Settings tab and then click the "Change" button connected to the Site Address.
You have three options when changing your site's address:
The first option is to Use a Sub-Domain of Weebly.com. This is free and allows you to create an address like myfantasticsite.weebly.com (anythingyouwant.weebly.com).
The second option is to Register a New Domain. There is a cost associated with this as you're registering your own domain name like www.example.com. The third option is Use a Domain You Already Own. If you've previously purchased a domain with Weebly, and are not using it with another site currently, you can enter it here. You can also use this option if you bought a domain through another registrar and want to use it with your Weebly site.