Navigate to /tomcat-root/conf folder. Within you will find the server.xml file.
Open the server.xml in your preferred editor. Search the below similar statement (not exactly same as below will differ)
<Connector port="8080" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="8443" />
Going to give the port number to 9090
<Connector port="9090" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="8443" />
Save the file and restart the server.
Now the tomcat will listen at port 9090