How to change an images source image from script? C#

I have been stuck on this for several hours now and I cant find any answer that I understand here or on google:/. Being a beginner at Unity and C# please try to explain for an idiot:)

I have my Image "testImage" decleared in my script and I have dragged the Image gameobject to the slot in inspector. In my project assets I have the other source image that I want to change to called "NewSourceImage".

   public Image testImage;
     
   public void Change()
     
     {
       testImage = GetComponent<Image>("NewSourceImage");
     }

I get the error message: error CS0308: The non-generic method `UnityEngine.Component.GetComponent(System.Type)' cannot be used with the type arguments