How to re-do the rotten underside of cantilever balcony

Hello, I'm trying to figure out how to replace this rotten, painted plywood on the underside of my cantilever balcony. I'm not worried about water coming down, so it doesn't need to be water-tight. I'd like it not to be, to ensure water doesn't pool or flow up to the side of the house. The underside of some of the beams underneath might be starting to rot, too, I haven't taken a good look underneath the plywood covering. Thank you