How do you change the ignition cylinder on a 1990 xj6 soverenge?

Not easy unless you have the key.

If you do, turn it to the accessory position and depress the pin at 1 o'clock on top of the cylinder body. Pull, and the cylinder should pull out of the body.

If you don't have the key, you'll have to remove the entire the entire cylinder body, then cut the cylinder out of the body by any means necessary.