You can use the Change event to do this. In the code module for
the appropriate worksheet, use code like the following. Change
the cell address to the cell you need.
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$C$3" Then '<<< Change to cell address
' your code here
End If
End Sub
Right-click on the sheet tab and choose View Code to open the
code module associated with the worksheet. To use a range of
cells, write code like
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Application.Intersect(Range("A1:C10"), Target) Is
Nothing Then
' do something
End If
End Sub