How to change a port for a specific IP address?

What are you really trying to achieve?

Something doesn't work and you want to fix it?

A different issue?

There is a manual for the C3700 somewhere at the end of this link:

>>>> C3700 | Product | Support | NETGEAR <<<<

By the way, you have posted your message in the section of this community given over to Apps and Services, which has nothing to do with the detailed workings of hardware. (This is easily done, given Netgear's impenetrable community structure.) Your device is a Cable Modem/Router.

You might get better replies, and find other answers, over in the appropriate section:

Cable Modems & Routers