Merging code for hiding columns and setting chart axes values

A few things.
Currently the cells you are checking have #REF errors. I have added error check to hide these. You might want to actually fix the formula.

You had no Next line.
No reference to the Axes object for setting values.

Sub Update()
Application.ScreenUpdating = False
    For Each cell In Range("AA1:DU1").Cells
        If Not IsError(cell) Then
            If LCase(cell) = "hide" Then
            cell.EntireColumn.Hidden = True
            Else: cell.EntireColumn.Hidden = False
            End If
        Else
            cell.EntireColumn.Hidden = True
        End If
    Next
    With ActiveSheet.ChartObjects(1).Chart
            ' Value (Y) Axis With .Axes(xlValue)
        With .Axes(xlValue)
            .MinimumScale = ActiveSheet.Range("$X$4").Value
            .MajorUnit = ActiveSheet.Range("$X$5").Value
        End With
    End With
End Sub