Hi
As far as I am aware there is no interface to access the actual windows components, menus etc.
2 methods come to mind.
1) Close Mach, then open NameOfYourProfile.xml in the ../Mach root directory.
Search for <StepsX>

</StepsX>, where X is the 0 based number of your stepper drive you wish to access.
Change the figure

to the new setting
Relaunch Mach
This could all be done through VB
2) Simulate a physical entry into the Motor Tuning menu, by using SendKeys() to output the simulated keystrokes necessary to reach the appropriate textbox and make an entry.
Since any change to steps/per will usually have necessitated a mechanical change, or more likely a change to the driver dip switch settings, it is unlikely to have been done with Mach running, so method 1 seems the best to me.
Not sure why you would need to do it, would be interesting to hear.
regards
Melee