Understanding of a value type and reference type

Using System. Collections. Generic;
Using System. text;
Using System;

Namespace Leleapplication1
{
Class Value
{
Public   Int I =   15 ;
}
Class Program
{
Static   Void Main ( String [] ARGs)
{
Program t =   New Program ();
T. First ();
}
Public   Void First ()
{
Int I =   5 ;
Value v =   New Value ();
V. I =   25 ;
Second (V, I );
Console. writeline (V. I );//20 ????? Don't understand depressing issues
  // Because in second, the value of V. I is assigned 20
// In second, V = val; V is changed to Val, but in this method, V is the original one.

}
Public   Void Second (value V, Int I)
{
I =   0 ;
V. I =   20 ;
Value Val =   New Value ();
V = Val;
Console. writeline (V. I +   "   "   + I +   "   " ); // 15, 0 understandable
}
}
}