syanjali is right, fill out 1040X (Amended Return). You will have to file Form 8843 as well. I am not sure if you would send that with 1040X or afterwards. May be someone else can advice you about 8843.

1040X Tax Return (http://www.irs.gov/pub/irs-pdf/f1040x.pdf?portlet=3)
1040X Instructions (http://www.irs.gov/pub/irs-pdf/i1040x.pdf)