Talk with her about other stuff
The first thing that comes to mind is that you should try to comfort her by talking with her like you would normally do. About normal everyday stuff, about hobbies, about news from around the world. Play games with her, read together, watch TV together - whatever it is that the two of you are normally doing together.
If that fails - talk about what the two of you are doing after the surgery
Just think about the future and talk about what events you might have planned for the time after the surgery. Maybe you two want to go to the park/cinema/... Again, whatever the two of you would normally do (and what is possible in the near time after the surgery).
All in all - make her think about the positive things
What will be better after the surgery? What will be easier? What can she do that she can't do now? What are problems she has today that she won't have anymore after the surgery?
Take her mind off of the immediate surgery. That only allows negative thoughts to occupy her mind. Fill that mind with positive thoughts about the time after the supposedly (in her mind) dangerous operation.
But: remember that sometimes listening is better than proposing a solution
It's important to be there for her. Maybe she just needs someone who will listen. It's not important to take her mind off of the operation she just wants to voice her concerns. Talking will make her feel better. Try to understand her and the problems and fears she tells you about. Don't try to solve them immediately by citing papers or saying that you have faith in the doctors.
Just listen, nod occasionally and say "Everything will be alright, you will see. And afterwards we are going to a date in the park."