Iām assuming your using ramps?
http://www.geeetech.com/wiki/index.php/Ramps1.4#Jumper_Instruction
So remove two jumpers. Just under the Z axis. Then change the steps_per_mm for Z to be half what it was. You can change it in the config (itās an obvious name STEPS_PER_SOMETHING), or using gcode:
http://marlinfw.org/docs/gcode/M092.html
If you screw it up, youāll be off by a factor of 2, 4, 8 so it will be a quick test to see if itās working.
Thereās been some debate about which microstepping to use. The tile of thumb I use is just to use 1/32 on an MPCNC, unless you have a reason not to. They are practically identical, but the common assumption on the forums is that youāve got it in 1/32. You wonāt actually lose any resolution because the steppers canāt really do a 1/32 step.