Geeetech

So I have a Prusa i3 X Acrylic. Impressed so far. Some build issues, and one of them was adjusted for in software.
The Y axis 0 switch mount position was too far back, so the part of the bed that hits it, triggers it about 35 mm behind where it should be.
Some friends adjusted it for me by making changes in the configuration.h area of the marlin software.

So here is where I am confused.
I am trying to adjust that back to original now that I have physically fixed the Y axis problem.
So until I fix this, prints that are supposed to be in the center are 35 mm back from center.

So my instructions were to load up the aduino software and then load up the marlin.ino file. This opens up what looks to be the same as the arduino window that I opened first.
So they have given me the file that is loaded on the 2560 board.

I have updated firmware on other things before but the arduino/marlin part has me confused.
Do I just make the changes on the saved file, and then upload it to the board through com 3?
Do I upload from the window that opens when marlin is loaded, or the first window?
When I tried this, I got an error when I tried to upload. Will check the error when I try again.
Would like a link to a basics on this if there is one.
Thanks.

So what't the error? You need to close the former arduino window and then compile and upload the firmware you need.
Make sure you've chosen the right board type as well as the serial port.