1) Go to: www.ipchicken.com
--> write down your current IP
2) Unplug your ADSL/Cable modem for 20 seconds (power cycle) and then re-connect... that should clear the current IP. If it does not then leave your modem unplugged for 3 to 5 minutes and try again - this should clear the MAC address stored in the modem and request a new IP from your ISP. The ISP may not assign a new IP, it will depend on there settings so I cannot guarntee this will work however with my provider Shaw, this works.
3) Go back to www.ipchicken.com
and see if your IP address has changed.
PS: from your post I gather you are trying to change the IP address at the ISP level and not your internal network... if this is not the case than post back with reply.