I'm not sure I understand the question, but you set the port the web server listens on in the Jetty config. In etc/jetty.xml you'll find something like this:
<Set name="port"><SystemProperty name="jetty.port" default="8080"/></Set>
You can run the server on a different port by changing that file or on the command-line when you start Jetty, like this:
java -Djetty.port=9090 -jar start.jar
If you want to use port 80, you will have to give the Java VM Administrator privs. This really has nothing to do with the supertweet local proxy webapp. It's generic to the Jetty web server (container) setup. Refer to these Jetty docs:
You could use Apache/Tomcat instead if you are more familiar with that.