Lets say ClassA is a class having one private member called "_Age" and objA the object of classA
Get private field value
object _value= objA.GetType().GetField("_Age", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(objA)
Change the private field value
objA.GetType().GetField("_Age", BindingFlags.NonPublic | BindingFlags.Instance).SetValue(objA, 20)