Found an error in a paper that I already presented at a student engineering conference; what should I do now?
For a paper published in a journal you would go for errata if the mistake is minor and does not invalidate the conclusions of the paper, or for retraction if the mistake invalidates the conclusions. In your case I would contact the organizers of the conference and ask what you should do.

You may feel ashamed about making a mistake that goes into publication (I talk from experience) but acknowledging it and trying to fix it is laudable (not everyone does it), is the ethical alternative and it will give you some peace of mind afterwards.