IMEI Change need help

Hi, I need to change the IMEI number of my phone because I bought my Nexus 5 from abroad in the Turkey you have to register the IMEI and pay for it I did but my phone got bricked and they changed the main board and I can't do anything to use it in Turkey please help I'm ready to root it and I have to use the IMEI of the old Mainboard that was registered in Turkey