how to change my ip ?

If you have a dial-up internet connectionn your IP address can change
frequently.  If you have a broadband (Cable modem or DSL) you most
likely have a static IP (one that doesn't change).  You can
temporarily change your IP by going through a proxy server (you can
google a list for free proxy servers) or you can contact your ISP
(internet service provider) and request an IP change or get a new ISP
all together.

depends most on your operating system.
assuming you have one of the windows variety, this is relatively easy
depending most on what your final objective is... want to clarify on
this?

one note on cable modem(s) - you do not have a static IP, you have an
IP that is consistently handed out to your MAC address. (this is the
hardware address that identifies your NIC, or Network interface Card)
dial-ups of most types will hand out new IP addys every time.

So, the easiest way to change this address if you are not on dial up
(which I will assume you are not or you would not have noticed the
'problem') is simply to buy a new NIC. prices are really cheap these
days, especially in multi packs.