You can change port in CapMonster settings.
But, after that you should send captchas from your software to CMipaddress:port instead of captcha-services (antigate.com, 2captcha.com, etc.).
Port 8888 has always worked fine for me. Just make sure no other program is using whichever port you choose to enter into CapMonster when you go to set it in CapMonster.