You have to scrape all the loose stuff off. I'd avoid pressure washing because a) it won't get all the peelings and dirt, b) it may chew up the surface, and c) the wood will be soaked and you'll have to wait days for it to dry out.
If it needs cleaned I'd do it by hand. I'd look also for any loose boards and secure those. Then just scrape the loose stuff, use a primer for any bare surfaces, caulk as needed, and paint a coat of finish. Or 2 finish coats if needed.
That fiberboard is famous for peeling paint. If you want, sand the bare spots with some light sandpaper [120 or 150] before you prime.
Also, you have to expect more peeling in years to come. Maybe it doesn't happen, but you have to expect that whatever made the paint peel this time will work on other areas of the old paint in time.