Can I modify a passed method parameter

You can change it but the change will not go back to the caller.

If it is a ValueType -> The copy of object is sent

If it is a RefernceType -> Copy of Object reference will be sent by value. In this way properties of the object can be changed but not the reference itself - caller will not see the change anyway.

If it is sent ref -> Reference can be changed.

In C++ you can use const to prevent the change but C# does not have that. This is only to prevent the programmer by mistake try to change it - depending where the const is used.

Changes to the value of a parameter are invisible to the caller, unless it's a ref or out parameter.

That's not the case if you make a change to an reference type object referred to by a parameter. For example:

public void Foo(StringBuilder b)
{
    // Changes the value of the parameter (b) - not seen by caller
    b = new StringBuilder();
}

public void Bar(StringBuilder b)
{
    // Changes the contents of the StringBuilder referred to by b's value -
    // this will be seen by the caller
    b.Append("Hello");
}

Finally, if the parameter is passed by reference, the change is seen:

public void Baz(ref StringBuilder b)
{
    // This change *will* be seen
    b = new StringBuilder();
}

For more on this, see my article on parameter passing.