OK, let's back up. Are you trying to change your router's IP address as seen on the internet? Because that IP address is assigned to you by your ISP and in most cases shouldn't be messed with. For a cable modem setup, just unplug the router and cable modem for a bit and then plug them back in. You should get a slightly different IP address. That address is called a public address.
Now, the private address that your laptop, iPhone, iMac, etc all see when they try to connect to the router...that's a private IP address and can be modified in accordance with the below table (from wikipedia). Your router should be the IP address at the start of the range.
IANA Reserved Private Network Ranges
Class Start of range End of range
The 24-bit Block A 10.0.0.0 10.255.255.255
The 20-bit Block B 172.16.0.0 172.31.255.255
The 16-bit Block C 192.168.0.0 192.168.255.255