Is it possible to change the functionality of a method by overriding it in a subclass?

Yes, in C# it is possible to change the functionality of a method by overriding it in a derived class. For this, the base method must be declared with the virtual keyword, and in the derived class, it can be overridden using the override keyword.

Example:

class BaseClass
{
    public virtual void ShowMessage()
    {
        Console.WriteLine("Message from the base class");
    }
}

class DerivedClass : BaseClass
{
    public override void ShowMessage()
    {
        Console.WriteLine("Overridden message from the derived class");
    }
}

Calling the ShowMessage method on an object of DerivedClass will execute the overridden version of the method.