In conversation I accidentally called what my girlfriend got me for christmas this year stupid. She told me, and she is mad. How do I apologize?

So we were walking around town and we saw a kid wearing Dr. Dre Beats and I made a comment about how they were unnecessarily expensive and stupid. She seemed hurt but I didn't realize why until later she told me that that was what she had gotten me from christmas and she stormed off. What can I do to make things right? How do I apologize?

tl;dr I called Beats headphones stupid and as it turns out she was planning on giving me them for christmas.