Yes, simply add (or edit) the following line to the configuration file (http://localhost:81/config_edit.html under Windows or http://localhost:2074/config_edit.html under Linux):  

webserver_port = 82


Where you replace 81 or 2074 by the chosen port. If you are running multiple instances on a single host, you can either set the same port for all instances, in which case each instance will automatically select the next contiguously available port (i.e. if you set webserver_port=100 in all instances, the selected ports will be 100, 101, 102, etc). 

Or you can set a different address of your choosing for each instance, in which case no automatic allocation will take place.