On a UserForm, a TextBox cannot sit above the FlexGrid control so you cannot easily place one over the grid to let the user edit. However, a Frame control can sit above the FlexGrid. Place a TextBox inside a Frame with no border and use that for editing.

When the user double clicks a cell or selects a cell and starts typing, the program calls subroutine GridEdit.

Private Sub flxGrid_DblClick()
    GridEdit Asc(" ")
End Sub

Private Sub flxGrid_KeyPress(KeyAscii As Integer)
    GridEdit KeyAscii
End Sub

Private Sub GridEdit(KeyAscii As Integer)
Const TWIPS_TO_PIX As Single = 72 / 1440

    ' End any previous edit.
    AcceptEdit

    ' Only edit if row > 0.
    If flxGrid.Row = 0 Then Exit Sub

    ' Position the TextBox over the cell.
    fraEdit.Left = flxGrid.Left + flxGrid.CellLeft * _
        TWIPS_TO_PIX
    fraEdit.Top = flxGrid.Top + flxGrid.CellTop * _
        TWIPS_TO_PIX
    fraEdit.Width = flxGrid.CellWidth * TWIPS_TO_PIX
    fraEdit.Height = flxGrid.CellHeight * TWIPS_TO_PIX
    txtEdit.Width = fraEdit.Width
    txtEdit.Height = fraEdit.Height
    fraEdit.ZOrder msoBringToFront
    fraEdit.Visible = True
    m_EditRow = flxGrid.Row
    m_EditCol = flxGrid.Col

    Select Case KeyAscii
        Case 0 To Asc(" ")
            txtEdit.Text = flxGrid.Text
            txtEdit.SelStart = Len(txtEdit.Text)
        Case Else
            txtEdit.Text = VBA.Chr$(KeyAscii)
            txtEdit.SelStart = 1
    End Select

    DoEvents
    txtEdit.SetFocus
End Sub

If the user presses Return, the program calls AcceptEdit to copy the changes to the FlexGrid and hide the TextBox's Frame.

If the user presses up or down arrow, the program calls AcceptEdit and moves to a new row. The program doesn't do anything special with left or right arrows in the TextBox because the TextBox already uses those to let the user move through the text.

Private Sub txtEdit_KeyDown(ByVal KeyCode As _
    MSForms.ReturnInteger, ByVal Shift As Integer)
    Select Case KeyCode
        Case vbKeyEscape
            ' Leave the text unchanged.
            fraEdit.Visible = False
            flxGrid.SetFocus

        Case vbKeyReturn
            ' Finish editing.
            AcceptEdit
            flxGrid.SetFocus

        Case vbKeyDown
            ' Move down 1 row.
            AcceptEdit
            If flxGrid.Row < flxGrid.Rows - 1 Then
                flxGrid.Row = flxGrid.Row + 1
            End If
            DoEvents
            flxGrid.SetFocus

        Case vbKeyUp
            ' Move up 1 row.
            AcceptEdit
            If flxGrid.Row > flxGrid.FixedRows Then
                flxGrid.Row = flxGrid.Row - 1
            End If
            DoEvents
            flxGrid.SetFocus
    End Select
End Sub

Private Sub flxGrid_EnterCell()
    AcceptEdit
End Sub

Private Sub AcceptEdit()
    If fraEdit.Visible Then
        flxGrid.TextMatrix(m_EditRow, m_EditCol) = _
            txtEdit.Text
        fraEdit.Visible = False
    End If
End Sub

Private Sub cmdCancel_Enter()
    AcceptEdit
End Sub

Private Sub cmdOk_Enter()
    AcceptEdit
End Sub

Private Sub flxGrid_Enter()
    AcceptEdit
End Sub

Private Sub flxGrid_Scroll()
    AcceptEdit
End Sub