However you changed the IMEI (and that's very difficult to do on any phone), get the number from the box the phone came in (if there are Dec and Hex numbers, you want the Dec number) or bring the phone into your carrier and ask them for it (be prepared to prove that it's your account with a photo ID). Can't really tell you how to change it, since only you know what you did to change it the first time. There's no button to tap to "get the original IMEI back". (It's not supposed to be changeable, even if you reflash the ROM.)