(I'm sorry for editing this. The answer that was here couldn't have been more wrong.
Short answer:
Stall speed REMAINS THE SAME with the change of altitude but Vmc DECREASES with an increase in altitude.
So at a higher altitude Vmc decreases and can be LOWER than Vs(stall speed) (AFH Figure 12-21) and adversely, as we descend, Vmc increases and can be higher than Vs at a lower altitude.
Long answer: Airplane Flying Handbook chapter 12.
-Daniel S. Murrill
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