Go to conf folder of your apache tomcat installation.
Open server.xml file.
Search for (Ctl+F) "Connector port" key word there. In the first matching occurence change the port value to the value which you wish.
For better accuracy make sure the current port value in the first matching occurence is equal to the port which the tomcat currently running.
Find the port which tomcat currently running.
Search for (Ctl+F) the current port in server.xml file and change it to the value you want.