If the cushion doesnt have a zipper, which most don't, you will have to cut the cushion open in a non conspicuous area, straighten the area out. You may want to add some layers of flat quilt batting, then stitch the area closed with a half circle needle and button thread. You can also do this to the arm areas with batting and just re-stuff them. It works good, I watched the repair man repair several. Good luck.
One more thing, if all else fails, you can take the cushion to an upholsterer, who can either fix it or place a new cushion inside the outer cover to get rid of the lump. It would cost less than replacing the sofa and may be worth the price due to your frustration over the problem.
Sometimes the inside of the cushion just twists, leaving what looks like a lump outside the cushion cover. Unzip the cushion cover, reach in, and smooth out the cushion inside, or if the inside cushion turned or rotated, turn it back where it's supposed to be (you'll have to do this slowly a bit of a turn at a time).
If the covers on the cushions have an opening in them you can buy foam rubber in different thicknesses, that can be cut to size when you purchase it. You will just have to know the size that you need. I don't know what all types of businesses sell this. Where I live, I know for sure there is one hardware type store that sells all thicknesses of foam rubber by the yard.
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