Which part is sagging?
Lift the cushion and add some boards to help support.
Might be time to buy a new couch if the other options don't work.
This post might help: https://handydiys.com/how-to-make-your-lumpy-couch-look-like-new/
If you have loose cushions, remove them and either place a long piece of MDF
on top of the couch and replace the cushions, or replace the foam in the cushions with fire retardent new foam or add a piece of MDF in the cushion with the foam on top. That should fix it.