How do I change something on my 1040 form? I was claimed as a dependent but I accidentally put in an exemption on my form.

If you have already filed the return and it was accepted by the IRS you'll have to file an amended return to make that change.  You will have to mail it in; amendments cannot be e-filed.

If you have NOT yet filed or if your return was rejected, you can go to My Info and change the way you answered the question about whether you can be claimed as someone else's dependent.