update a cell using VBA code example

Example 1: vba set cell value

Range("A2").Value = 1
Cells(2,1).Value = 1
Range("A2:A5").Value = 1    ' all cells in the range

Example 2: excel vba cell on change

' In your Worksheet code:
Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Intersect(Target, Target.Worksheet.Range("A1")) Is Nothing Then 
    	Application.EnableEvents = False
    	'  Do your stuff
    Application.EnableEvents = True
    End If
End Sub