How do I unlock my iPhone with my IMEI code on iTunes. And how do I change the iCloud account connecting to the phone?

My sister got the 5S, so she has given me her old 4S. She got the IMEI code from her network to allow me to unlock it. When I plug the phone into my computer, how do I use the IMEI code to unlock it on iTunes. The text my sis got from O2 says, "Apple have confirmed the iPhone IMEI 01.....etc. is set to unlock. To complete, connect the device to iTunes with a SIM never used in the iPhone". But I'm not sure what to do, I already connecting the iPhone with my SIM to iTunes... What next? Also how do I change the iCloud account connected to the phone (hers) to my own?

Thanks.