One way:
Put this in your worksheet code module (right-click the worksheet tab
and choose View Code):
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
With Target
If .Cells.Count > 1 Then Exit Sub
If .Address(False, False) = "B2" Then
If IsNumeric(.Value) Then
On Error Resume Next
Application.EnableEvents = False
.Value = -.Value
Application.EnableEvents = True
On Error GoTo 0
End If
End If
End With
End Sub