If it's just the cable you need to replace, it should be pretty straightforward. Pull the headliner off the sunroof, unbolt the cable from the cross-brace, run the sunroof motor all the way forward until the cable stops coming out, then pull it the rest of the way.

Installation is basically the reverse (lube the cable if it didn't come that way already). Push the cable in (you should hit a little bit of resistance two or three times, where the cable housing bends) until it stops, then run the motor while back while pushing on the cable (to make sure it engages) until the bracket lines up with the sunroof cross brace, then bolt it back in. I can't remember if you need to retime the motor after this.

If you have to remove the cable guide housing, that is a completely different story.