The above will work, but since a Do loop is used, it may not look at all cells that you would like it to. For example, if one of the cells in column B is blank, but there is data below it, the Do loop will not work. In this case, a For Each loop will work better, which will look at each cell in column B that has a value:
Sub Notes()
    Dim Cell As Range, Rng As Range
    Dim response
    Set Rng = Range("B1:B" & Range("B" & Cells.Rows.Count).End(xlUp).Row)
    
    For Each Cell In Rng
        If UCase(Cell) = "SEE NOTES" Then
            response = InputBox("Cell " & Cell.Address(False, False) & " Notes:" & _
            vbCr & Cell.Offset(0, 1) & vbCr & vbCr & "Enter updated value:", "Notes")
            If response <> "" Then Cell = response
        End If
    Next Cell
End Sub
HTH

Jason