I would take a backup of the content database(s) from the web application, create a new web application with the correct URL, and restore content databases to the new web application.
Microsoft explains a little more complex:
After you have extended a web application into a zone with a set of Internet Information Services (IIS) bindings and alternate access mapping URLs, you might decide that you want to use a different URL to reach the web application. For example, you might have originally created a web application to use HTTP and later decide to use SSL instead. Or, you might create a web application to use the www.contoso1.com host header and then decide to use the www.contoso2.com host header instead. This article provides detailed guidance for changing the URL and IIS bindings of a web application.
Unlike typical IIS applications, you cannot simply use IIS Manager or other IIS metabase tools to modify the bindings of IIS web applications that have been extended with SharePoint 2013.
If you modify the IIS bindings of a web application by adding a host header binding or SSL port or by changing a port number, SharePoint 2013 will not be aware of these changes and will not update the web application's alternate access mapping URLs. If you update the web application's alternate access mappings to change a host header, switch to an SSL URL, or change a port number, SharePoint 2013 will not automatically update your IIS bindings to match.
To update the URL or IIS bindings of a web application, unextend and reextend the web application and reconfigure the alternate access mapping URLs or IIS website bindings.
We do not recommend reusing the same IIS website for your HTTP and SSL hosting. Instead, extend a dedicated HTTP and a dedicated SSL website, with each assigned to its own alternate access mapping zone and URLs.
Source: Update a web application URL and IIS bindings for SharePoint 2013