I don't see any way of changing it in the GUI.. maybe you could try this...
You can locate the web.config file where you installed FileMaker..
C:\Program Files\FileMaker\FileMaker Server\HTTPServer\conf
Its not as simple as changing the LISTEN directive in HTTPServer/conf/httpd.conf.
When I do that (e.g. to port 4180), the browser gets redirected to;
I think its due to the rewrite rule in HTTPServer/conf/extra/httpd-rewrite-fmiwebd.conf, but haven't had more time to work on it yet.