I’m assuming your using ramps?
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 (it’s an obvious name STEPS_PER_SOMETHING), or using gcode:
If you screw it up, you’ll be off by a factor of 2, 4, 8 so it will be a quick test to see if it’s working.
There’s 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 you’ve got it in 1/32. You won’t actually lose any resolution because the steppers can’t really do a 1/32 step.