1) Locate server.xml it will in the conf folder in the tomcat installation folder
(Tomcat installation folder)\conf\
e.g
I have it as
D:\Java\apache-tomcat-6.0.28\conf
2) In this folder there will be a file server.xml
Open this file in notepad and change the port number to what ever you want.
<Connector port="8080" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="8443" />