Delphi String Replace Function takes 4 parameters. first parameter string to be changed 2. parameter string to change 3. parameter to be replaced by string to replace. 4. The parameter is in array type. specifies additional features for the change.
 

var
 input , output: string;

begin
  input := 'Hallo World Laarning Dalphi';

  output:= stringreplace(input , ' a ', ' e',  [rfReplaceAll, rfIgnoreCase]);
  ShowMessage('first= '+input );
  ShowMessage('after= '+output);
end;

output:

first= Hallo World Laarning Dalphi
after= Hello World Learning Delphi