What are the variables for PID control? How to use M301? How to use this command to switch from PID to bang-bang?
What command would I enter to make this a bang-bang PID controller


There's no such thing as a "bang-bang PID controller". "bang bang" is mutually exclusive of PID. The M301 command is only good for fine-tuning the parameters of the PID controller, but it won't let you switch to bang bang. Unfortunately, you must update the firmware if you wish to use bang bang.

You could try setting all values to 0, except P, which you set to an as high value as possible. This will cause the output to be fully on when the temperature is lower than the setpoint, and fully off when higher than the setpoint. However, this can still cause rapid clicking of the relay when the temperature is hovering around the setpoint. Bang bang has some hysteresis built-in (i.e., there is a margin around the setpoint in which the relay will never be toggled); there's no way to emulate this with PID.