Hi Pat

Here are some answers for your project.

https://www.hunker.com/13403693/how-to-replace-the-foam-in-attached-cushions

https://www.hometalk.com/29949640/q-i-need-to-restuff-my-non-removable-sofa-cushions

The sides seem to be less noticeable if you are going to cut open and resew after re-stuffing.

I agree with Ken - find a hidden seem in the back, cut the stitches and fill it that way. Then use upholstery strength thread to sew it back up.

Pat: Maura and Ken are correct! ...that's exactly what I'd do. (...and you do NOT need to open it the entire way... to fill it)

It has to have a hidden seam or else it would not have been stuffed to begin with. Go to the back and carefully cut open the seam, it maybe hard to get to, have patience. Only open a small enough are to add stuffing to your desired thickness, it will pack down again over time. Resew the opened stiches with upholstery thread. You can also buy curved needles to make this project easier.

If you don't have a zipper, if you have any sewing skills, you could open the seam and add a cut to size mattress topper but then you would have to hand sew it back. I use them in my upholstery projects as well. I used a mattress foam topper in this project that I was re-upholstering because the existing cushions lost their oomph. Here is the post if you are interested...

https://karupp-did.net/katies-rockin-rocker/

If you don't care about cost though and you want it to be easier than having to cut foam to size, I did find a product right off amazon but you would have to do the same thing as I described. Open the seam and insert this. Sew back up.

https://www.amazon.com/Industrial-Strength-Customizable-Furniture-Appearance/dp/B01CYREI4A/ref=asc_df_B01CYREI4A/?tag=hyprod-20&linkCode=df0&hvadid=191941654061&hvpos=1o5&hvnetw=g&hvrand=10467516976199535211&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9007919&hvtargid=pla-303155001581&psc=1

You can unpick one of the seams and add stuffing then sew it back up with a hidden stitch.

Do you want the cushion insert changed or just add filling? Pick an area you can hide a seam or reopen a seam. Use upholstery type stuffing or foam rubber for the best result. Depending on which you decide on, will determine the size of your opening. For fiberfill stuffing,

, you can shove it in by the handful, pushing it firmly in the area furthest from you, working towards the opening. In dollmaking, I use forceps, a chip stick, a pencil eraser, or a skewer to stuff it in firmly and so can you. A dowel works, too. Gold luck!

Hi Pat. If there is no seam on the seat cushion, you might have to go in from the back. The backs of most sofas are stapled down. You can remove these upholstery staples with a screw driver. This should give you access to the cushion from the back. A few years ago I had a new sofa that needed repairs. I watched how he pulled out part of the back. Did the repair and stapled the back into place.