function Update in edit mode

How can I use a function update in scene view, not play mode? More specifically: To make an editor window, a static function has to be used. And you can't use an update with one of those. I can't just add @ExecuteInEditMode to the top, because this makes it work very very slowly. I want to make a scriptableWizard, but can't work out how to do it. So I need to work out how to all the function Update. If you want to look at the script, it's here.