Getting around a BAN

i was wondering I has cable modem and was wondering how I change IP address on it from my computer which is 2 routers away from main connection or do I just change the IP for my PC

its a fucked up network that really isnt a working network like u think nothing is connected to anything other then my broadband for nets i have 3 computers and 1 router hooked into main router and that other router is hooked up to my pc and my 360 and PS3 thats the setup in a nutshell unless my sister is home and she runs off the main router which is also a wireless