1. A rooted phone
2. Xposed framework/Xposed installer
3. Xposed imei changer
NB: There are so many tutorials on how to root your phone in this forum. Wouldn't waste time on it. For samsung users, you can use odin software. It's very easy to use.
You can get Xposed Framework on your phone by grabbing the Xposed Installer app. Just download it using this http://dl.xposed.info/latest.apk, which will give you latest APK straight from the developer of the app. Once the download is completed, install it like you would any other app.
Step 2: Update & Reboot
After installation, open up Xposed Installer and head to the Framework section and select Install/Update to ensure that we have the latest version.
After the update installs, Soft reboot or Reboot your device to enable Xposed.
How to Install imei changer
Enable module in xposed framework
Come back to app and enter new value
Go to xposed module
Open framework page
Do a soft reboot
Open the app
And you can see new value
link : https://d7.downlod.me/d/7ed4kfsokqleghircjuy553gxcizmq4jyevskxwfnkcwxbflio2gkgl2/IMEI%20Changer%20Pro_1.2.apk
NB: MOST SAMSUNG PHONES REQUIRE THAT YOU DEODEX THE STOCK ROMS SINCE MOST ARE ODEXED. IN SUCH SITUATIONS JUST FLASH YOUR PHONE WITH A CUSTOM ROM. LEMME KNOW IF YOU ENCOUNTER ANY PROBLEM.
NB:Changing a phone's imei is illegal. This forum or myself wouldn't be held for any problems you encounter doing this. It's for educational purposes only.