How to assign value to a constant or static class property

I wrote a test code to set a Static property (Constant, as recommended) in MyClass, as TestStatic. In TestStatic I call a static method in MyClass, setA(), to change the value of one of the constant properties. When I stop then pass over the line

MyClass.m_a = aNewA;

I can see that the variable changed value. But after I step out from the function setA(), the value returned to the default.

May I ask what I did run and what should be the fix?

In the test code, the program should start from TestStatic.

Thanks.