Firstly we need to modify the ssh configuration file named /etc/ssh/sshd_config.
The location of the file is: /etc/ssh/sshd_config
You will need to open the file using your favorite text editor and find the line:

#Port 22

The default port for SSH is 22 and # is there because the line is commented. If you want to change this port you need simply to uncomment the line deleting the # sign in front of it and if you want to change it to 12134 for example the line should looks like this:

Port 12134

Once this is done you will need to restart the SSH daemon service. Here is how it should be done:
In the most linux distribution you can do it using the command:
root@somebox ~$ /etc/init.d/sshd restart

In FreeBSD this should be done using the command:
root@somebox ~$ /etc/rc.d/sshd reload

Now you can login over SSH to the new port and the old one won't be active for SSH connections anymore.

Have fun.