Can't edit a listing

I have a current listing and I would like to edit the listing. I want to add a sentence. But there is only an option to 'edit listing options' or 'withdraw or mark listing as sold'. I just want to add a line in there. But I can't get to that screen. I've been at this all day....it just won't work on my pc. I don't have the app. I'm using a microsoft computer.