'If u are looking for a macro to do that below is the code
but be aware it converts blank to zero
Sub macro2()
Dim k As Range
Set k = Application.InputBox(Prompt:="Please select the range",
Title:="Specify range", Type:=8)
On Error Resume Next
If k Is Nothing Then
Exit Sub
Else
For Each cell In k
cell.Value = -cell.Value
Next cell
End If
End Sub