I have egregiously sloppy (possibly falsified) data that I need to correct. How should I go about this?
I infer from the question that you still have the raw data. In that case, I think you need to re-do all the steps in which errors could be introduced (such as data entry, spread-sheet calculations, analysis of trends). Then see what effect the corrections have on the conclusions of your paper, and rewrite as necessary. I realize that this sounds like a lot of work, essentially repeating most of what you had done earlier, but since you apparently did it wrong the first time, you now need to do it right.
Meanwhile, you should tell the PI about the problem and your plans for correcting it. If I correctly understand the situation, the raw data are the only reliable information currently available. You should make sure he knows that, so that he (and others working with him) won't build further research on a foundation of wrong results.