I am guessing you will do the routine prep for product/case study round. Be prepared for a short case or guestimate. At APM level, key skills they will expect would be problem solving, analytics, data orientation, and ownership.
Read up about the company- website, published articles, interviews with founder, if any, where they might have talked about company vision/strategy. Go through their product - app/website. Understand the product as much as you can. It's great if you can form your view about it - what could be improved from a customer's point of view. All of this is not very critical for an APM role, but it will be a huge positive if you are able to show some insights. For some founders, whether candidate has done this homework and if they are genuinely interested in the company or not is important.