You should be able to unzip the cover to the cushions. Take the cushions out & simply wrap some quilt batting around them. Stuff them back into the covers & you should have some plumper cushions!
Hope that helps!
If there's a zipper it's easy. If no zipper, I would very carefully slit a back or side seam, slip in the stuffing into the case inside or under the upholstery. Then use a matching threaded needle (I double thread or use button thread) to whip stitch or hidden stitch the slit. The slit doesn't have to be enormous, just big enough for your hand and stuffing to fit. I've used quilt batting before, making sure it is not lumpy 'cause it will show. You could slit an entire back seam, laying a really fat piece of batting also but, it just depends on how much time or willingness to stitch it closed. You could use a spray adhesive on poly foam rolls before you inset it, it will move around a little to get it right. This will help the stuffing stay in place. Always take your time so your happy with your finished product.
You certainly can restuff a couch cushion. If the cushion is attached to the couch, you will need to cut a slit as far back on or under the cushion as you can, preferably at a seam. Push the current stuffing toward the front of the cushion and into the front corners. Then place the new stuffing in, handful by handful. If you have an old baggy pillow, you can stuff that in with more stuffing around it. When the cushion is as full as you want, sew the slit back together. Continue with other cushions. If the cushions are loose from the couch, follow the same directions, but it will be much easier than attached cushions. Best wishes! 😊
Shirley,. Whatever your stuffing is now purchase the stuffing the same. You can get this in a craft store or furniture store. Open the zipper and stuff it until you feel it's firm enough. Aloha!