PID Calibration on the Core One  

RE:

You can run PID calibration however it has to be done by sending g-code to the printer. You will need to connect to your computer to the printer over the serial USB-C port in the back,  open a terminal like putty, screen, cool term etc. And send the g-code command to initiate PID calibration: M303 S250 (sets hot end target to 250C). When it completes you will see (for example):

 Classic PID 
 Kp: 30.77 Ki: 2.87 Kd: 82.45
PID Autotune finished! Put the last Kp, Ki and Kd constants from below into Configuration.h
#define DEFAULT_Kp 30.77
#define DEFAULT_Ki 2.87
#define DEFAULT_Kd 82.45

You will then need to add these to the custom g-code section of your printer under Start G-code as (again for example) since there is no facility to store this in the firmware. 

M17 ; enable steppers
M862.1 P[nozzle_diameter] A{(filament_abrasive[0] ? 1 : 0)} F{(nozzle_high_flow[0] ? 1 : 0)} ; nozzle check
M862.3 P "COREONE" ; printer model check
M862.5 P2 ; g-code level check
M862.6 P"Input shaper" ; FW feature check
M115 U6.3.4+10511

; Calibrated PID
M301 P30.77 I2.87 D76.09

RE: PID Calibration on the Core One

Another way to connect to the C1 is via OctoPrint which is what I use. It works best if you have a RaspberryPi.