How do you amend your taxes that you filed this year?

I filed my taxes for the 2007 year in February. I already received my return. I have two unclaimed jobs that I would like to use as deductions or whatnot. How do I go about doing this? I went to HRBlock to get my taxes filed. Do I need to go back there and explain to them what happened? That I was a bit impatient that my W2