How can I repair a sagging recliner?

The foam or the frame is broken down?

In either case, I’d start to take it apart in the least visible place. Maybe the bottom, and/or the back.

I’d use a large seam ripper (sewing store) and a screw driver to pry any staples up.

Cushions can be re-stuffed in-place with loose batting. Foam can be cut, compressed and inserted too.

If the frame is cracked, then buy large furniture clamps, wood glue and glue it back together with some reinforcement screws. I would pre-drill pilot holes for the screws.

It's the foam that's broken-down. The frame is solid. So, loose batting from a sewing store, or foam (where would I get that?)