As stated in documentation:
The Siebel Connection Broker (alias SCBroker) component is a background-mode server component that provides intraserver load balancing. By default, it is always enabled and online. At least one instance of SCBroker must be running on any Siebel Server hosting interactive components.
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SCBroker listens on a configurable, static port for new connection requests from the Web server or a third-party load balancer. The parameter, Static Port Number (alias PortNumber), defines the port that SCBroker monitors. The default value for this parameter is 2321.
So you can change mentioned "PortNumber" parameter using srvrmgr utility (Siebel Server Manager). Instructions how to use this utility are placed here. In short, you should connect to your server with the following command:
srvrmgr /g your_gateway /e your_enterprise /s your_siebel_server /u sadmin /p password
Then to verify current value of PortNumber parameter:
srvrmgr> list param portnumber for comp scbroker show PA_VALUE
To change parameter execute:
srvrmgr> change parameter portnumber=12345 for component scbroker
Where "12345" is new SCBroker port number. After that you need to bounce the component (note that server will be unavailable for users during restart):
srvrmgr> shutdown systemcomps
srvrmgr> startup systemcomps