From the sounds of it, you probably can't - this is something which would typically be controlled/assigned by your ISP - so you would need to speak to them about it. Depending on your ISP and the reasons behind the change, you might find that if you switch your router off for an extended period of time (many hours), you may be assigned a different IP address.
I do note that this change may not be innocent. If you are doing questionable things on your Internet account and your ISP has decided to monitor it more closely, it is entirely probable they have assigned you a static IP address to make their lives easier.
Of-course, it is entirely possible they have changed their IP address allocation policy - there are some technical benefits to clients to retaining the same IP address where possible, and with the advent of always-on connections the opportunity cost of doing this is quite small.
Is there any software or something like that?? – Aashish Kumar Shaw – 2015-10-10T08:27:59.873
1No offence, but you really need to get some critical reasoning skills. I provided the answer to this in the first sentence of the first paragraph of my response. There is software you can use to change your apparent IP address, but that will not change the address the ISP provides you and comes with costs and/or technical penalties such as performance. You can use a VPN, Tor or maybe a proxy server. You can change ISP's. – davidgo – 2015-10-10T08:34:57.763