How would I go about dropping it easy or getting a FWB?

So last night me and some other people spent the night at my friends house. There were more girls than guys and I ended hooking up with one of them (im a guy). I didn't expect myself to hook up with her because we dont really talk much at school, but we did and now I am pretty sure she wants me. After I left my friends house in the morning, she texted me a few hours later thanking me for keeping her warm last night. I don't really like her like that, so what could I say to let her down easy or turn her into a friend with benefit?