It's hard to give you any specifics with so little to go on. But I would recommend a technique called "pace and lead", something you can use anytime with anyone. It is related to "the law of state transference" from RSD and is about simple (positive) manipulation. Even though it is called NLP, I think it such a natural thing to do that you can as well just call it "social competence", plain and simple. Here are a few links to read (I haven't read them myself though):
http://nlptraining.com/articles/pacing-and-leading/
http://www.pickupguide.com/layguide/pacing.htm
http://hilgart.org/enformy/dma-pace.htm
Pacing means that you meet her at the same "energy level" so to speak, or the same mood or mindset. You can start synchronizing your body language, your tonality and the way you talk with her in order to build rapport. Just imitate her, but be careful to not be too obvious - she mustn't realize what you are doing! If she changes position in the chair, you can wait 30 seconds until you follow her and do the same.
When you have imitated her for a while (a couple of minutes), being that understanding and good listener, it is time to LEAD. Now you make some slight changes in your body language toward a more open (no arms crossed) and confident posture. Try to increase the energy in the way you talk towards a more happy mood, etc. Hopefully she will follow you unconsciously. If not, you have to fall back to pacing again.
About changing the topic... After you have listened to her for a while, just say something like "let's not talk about this anymore, let's talk/do something fun instead. ". And then you take the conversation wherever you want it.
CHAT