To calibrate the initial distance between extruder and platform. After homing completed, slide a piece of paper continuously back and forth between the nozzle and the build plate and simultaneously manually tap up and down arrow to adjust the distance until the paper causes a slight friction.
Up arrow: Extruder will elevate, away from the platform;
Down arrow: Extruder will descend, move close to the platform;
Tap OK to complete calibration.