Replaces selected text
#include <GuiRichEdit.au3>
_GUICtrlRichEdit_ReplaceText ( $hWnd, $sText [, $bCanUndo = True] )
| $hWnd | Handle to the control | 
| $sText | Replacement text | 
| $bCanUndo | [optional] Can operation can be undone? True (Default) or False | 
| Success: | True. | 
| Failure: | False and sets the @error flag to non-zero. | 
| @error: | 101 - $hWnd is not a handle 103 - $bCanUndo must be True or False -1 - no text is selected  | 
_GUICtrlRichEdit_AppendText, _GUICtrlRichEdit_InsertText, _GUICtrlRichEdit_SetText
Search EM_REPLACESEL in MSDN Library.
#include <GUIConstantsEx.au3>
#include <GuiRichEdit.au3>
#include <WindowsStylesConstants.au3>
Example()
Func Example()
    Local $hGui = GUICreate("RichEdit Replace Text (v" & @AutoItVersion & ")", 340, 350, -1, -1)
    Local $hRichEdit = _GUICtrlRichEdit_Create($hGui, "This is a test.", 10, 10, 320, 220, _
            BitOR($ES_MULTILINE, $WS_VSCROLL, $ES_AUTOVSCROLL))
    Local $idLbl_Msg = GUICtrlCreateLabel("", 10, 235, 300, 60)
    Local $idBtn_Next = GUICtrlCreateButton("Next", 270, 310, 40, 30)
    GUISetState(@SW_SHOW)
    _GUICtrlRichEdit_SetText($hRichEdit, "First paragraph")
    Local $iMsg, $iStep = 0
    While True
        $iMsg = GUIGetMsg()
        Select
            Case $iMsg = $GUI_EVENT_CLOSE
                _GUICtrlRichEdit_Destroy($hRichEdit) ; needed unless script crashes
                ; GUIDelete()   ; is OK too
                Exit
            Case $iMsg = $idBtn_Next
                $iStep += 1
                Switch $iStep
                    Case 1
                        _GUICtrlRichEdit_SetSel($hRichEdit, 0, 5)
                    Case 2
                        _GUICtrlRichEdit_ReplaceText($hRichEdit, "1st")
                        GUICtrlSetData($idLbl_Msg, "Text has been replaced")
                    Case 3
                        _GUICtrlRichEdit_SetSel($hRichEdit, 0, 3)
                        _GUICtrlRichEdit_ReplaceText($hRichEdit, "")
                        GUICtrlSetData($idLbl_Msg, "1st has been deleted")
                        GUICtrlSetState($idBtn_Next, $GUI_DISABLE)
                EndSwitch
        EndSelect
    WEnd
EndFunc   ;==>Example