From the KK2.1 V1.18S1 Manual over on *** in Steveis's blog:
o Set your transmitter throttle stick to maximum
o Press and hold down buttons 1 & 4 on the KK2.1.X
o Turn on the power to the Multicopter
o Wait for the ESC to beep its full throttle calibrated signal. Takes a few seconds, depends on the ESC
o Lower the throttle to idle
o Wait for the idle throttle calibrated signal
o Release buttons 1 & 4.