Fran,
Do not force anything unless your bent on spending much dollars to replace it. To remove the tension adjusting knob all you need to do is push the numbered dial "in" and then turn the knob counter clockwise. It will come apart.
Now, you didn't say what style of top tension your machine has, or weather you just want to take the dials and disks off to redo them or remove the entire top tension.
To remove the entire top tension it requires you to take off the face plate and loosen a small grub screw. You'll see it, it's the only one near the tension.
Your owners manual has instructions on how to do much of this.
Joe