What should I do if I accepted a review request based on paper title, but then found out it touches on topics I don't know?
You have two choices, each of which I have done at different times:


You can tell your advisor and the editors that now that you've seen the paper, you no longer think you could do a good job reviewing it. 
You can review it to the best of your ability and state clearly in the review that you have low confidence in your review. (Often, this can be indicated numerically.)