Please help! What should I do before unrooting?
It's a simple answer, but when I used that program, I did not lose data by unrooting. To unroot, go to Settings, and press "Unroot". You will see a "com.kingroot.kinguser has stopped" message, and your phone will reboot with its data still intact.

For more see If I root my Android phone, will I lose all of my data?