How do you re-stuff a chair cushion that is attached to the chair?

Like this ?

Add a cushion on top.

So is this a chair where the cushion is on the seat and upholstery is holding it in place to the chair? Is it possible to provide a photo of it?

I think you’d have to cut the fabric in an inconspicuous spot and stuff with some stuffing and re-sew.

I have recovered cushions that are stapled on such as dining room chairs. You need to remove the staples, recut a new upholstery fabric (you can use the one you remove for the pattern shape) replace the foam (you can have a new piece cut for you at a foam store) and use a staple gun to reattach. Its really not difficult at all.
https://www.youtube.com/watch?v=rj3Nzp5IjuY

It might be a fully upholstered arm chair where the seat cushion is separate, yet the bottom is attached to the seat of the chair in order that it doesn’t slide out with use. For example, some recliners do not have removable seat cushions.

It depends on the age, quality and value of the chair.

Most modern furniture is meant to be disposable, and the value of used upholstered furniture is so low that buying new or new-to-you is probably the best route.

Thanks to all for your suggestions. After full examination of my chair, i’m going to cut into the cushion and add memory foam. The chair is definitely worth it.