NicoleC, as you are close to the performance time, it might be a problem. As inncreasing your range takes time, practicing correctly each day and gradually training the muscles involved, you need professional coaching starting now. It's like running a marathon.. you have to build up to it, not just get up one day and do it. I agree with the suggestions to talk to your conductor first and hopefully you will get individual instruction there for exercises. You'll strain your voice by trying to "get there" too quickly, pushing against your vocal cords.

If you can't find a teacher or help from your instructor, you can get a personalized exercise tape/CD from my voice teacher at an affordable cost, $20. Send me a PM and I'll give you the info to get that done.

Alice