We need more information from you to troubleshoot further. What type of machine do you have (MPCNC?) and what controller are you running on that machine?
That’s not really a thing. 
You might be running GRBL or GRBL32 on some controllers, but since you say you want to run $SS, then I’m going to assume you’re running FluidNC (Which is a derivative of GRBL32).
For these boards, the best option is to use the web UI and work with the config.yaml file which defines your machine. That’s where you change steps/mm (and every single other setting)
On FluidNC, you change the config file, you don’t have to modify the firmware unless you upgrade to newer versions of FluidNC.
It’s like Dan said in your other thread:
Edit to add specifics:
What machine type do you have?
Which model of controller board is in your machine?
(Post pictures if you don’t know)
How did you load the initial configuration on the controller that you have?