If you have an upholstered sofa with cushions permanently sewn onto the fabric, you may need to remove the cushions at some point to replace the cushion fabric or replace the filling inside the cushion. Expect the process to be tedious but not overly difficult. Once you get started, you will see that the project is straightforward.
Lift up the cushion as far as you can to expose the stitching that secures the cushion to the top of the fabric for the main portion of the sofa.
Cut a thread used to stitch the cushion to the couch. Fingernail clippers work well for this because you can use the edge of the cutter to cut the thread.
Pull the thread out with a pair of tweezers until you remove enough of the thread that you can pull it with your fingers. Continue to do this as you work your way around the cushion.