how java string immutable?

I have a string in method in java class as below

public void show(){
  String s1;
  s1 = "abc";
  s1 = "def";
  System.out.println(s1);
}

Here the output is def, but as String is immutable so I don't understand how string s1 is immutable here as I can change the String s1 content from abc to def.

Could you please make me understand this?