Boxing vs Unboxing

Question

Another recent C# interview question I had was if I knew what Boxing and Unboxing is. I explained that value types are on Stack and reference types on Heap. When a value is cast

Answer 1

Very short: boxing means creating a new instance of a reference type. If you know this, you understand that one instance does not change by creating another.

What you are doing with a = 2 is not changing the value in the 'box', you are creating a new instance of a reference type. So why should anything else change?