Code:

SendMessage(Memo.Handle, EM_REPLACESEL, 0, PCHAR('Delphi World - это КРУТО!'));

Var TempBuf :Array [0..255] of Char;

SendMessage(Memo.Handle, EM_REPLACESEL, 0, StrPCopy(TempBuf,'Delphi World - это КРУТО!'));

Memo1.SelText := 'Delphi World!';