Flipping positive Y

Running a custom X carve, basically. My Y positive direction is in the "wrong direction" and was wondering if anyone could point me in the direction of where in the firmware to change this... it's the last big problem keeping me from running the machine... Arduino Uno, GShield, UGCS

Edit: a little light on details, sorry:
due to the shop setup, I want to have HOME in the back right corner, and the machine is currently set that way. However, It thinks that positive Y is in the wrong direction, ie right instead of left onto the bed.

And if someone could confirm/deny that the first setting in UGCS is what I might change to set my steps-to-mm ratio?? $0 is pulses for steps, in microseconds? Is there a better way to change my steps-mm ratio maybe in the firmware? My X and Y axis are doing 140mm instead of 25, Z is 19/25.

Edited 1 time(s). Last edit at 03/16/2018 09:48PM by PBZ.

TBH, I don't know much about the Gshield, but in general:

To flip the Y-direction, you have to revers the stepper connector or revers it in firmware. Also you have to place the endstop to the other end of the mill or declare it "the other way" in firmware too. ( min-endstop becomes max. and vice versa ) Make sure, the endstop cable is then placed in the right socket.