Changing port of existing web application

Two step procress.

First in Central Administration navigate to Application Management \ Configure Alternate Access Mappings

In the Alternate Access Mapping Collection dropdown select the required web application

Click Edit Public URLs

Change port as required in default and any other existing URLs

Then in IIS Manager

Select the site in the Connections pane

Click Bindings

Change port bindings as required

Note: This does not update the IIS bindings: "If you update the alternate access mapping URLs of a Web application, SharePoint Foundation 2010 does not automatically update the IIS bindings." (Technet). Therefore, you must also change the port in IIS.