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For this parameter (#steps/mm) for a given axis (Z in this case), it's not necessary to get into the firmware for now.
Using a software like Repetier Host or Pronterface or Cura…, you have to connect your printer and type :
M92 Znnn

where nnn is the appropriate value for your new configuration

M500 tells the printer to record/memorize this value.

That's all (for now).