Hello, recently my ip address was changed and i don't know why.
Does anyone know how to change ip address ?

Do you mean your IP in your LAN, or your IP on the internet? Most ISP's don't allow you to change your IP-address. They are sometimes dynamic and thus can change at any time.

Yes ip in my lan, i know is dynamic but why was changed and now didn't want to change again ?

When your PC is turned off the DHCP server (modem/router) will see the address is free and it will then assign the address to another device that enters the network. When you enter the network again, the DHCP server will assign you to the first available address.

It obviously is more complicated than I describe, buy that's how it can work.

You can set a static IP address, so whenever you turn off your computer.. The IP will be the same. Don't forget to lock the IP address on the router.

EDIT: Its not as simple as it sounds.