How to approach a mistake I made in an abstract for a Conference? In about a month I will be presenting a poster among 100+ people in an international congress. However, I'm just realizing that I made a huge mistake in the abstract I sent (accepted of course), concerning a whole phrase (15 words) with numerical results. This doesn't change my overall conclusion, because combined with the other numerical results that I offered (which are fine, no mistakes), I reach the same observations. 

I'm a last year undergraduate student in STEM, and I'm about to tell my advisor hoping for the best. But before I tell them, should I be prepared to send a correction to the organizers? I've been browsing the stack for a while and people seem to be super light on posters at conferences, so what are your thoughts?
Yes, send a correction, with apologies. It may be that they can make a change or not, but "due diligence" says you let them know. You might be prepared for the poster session with a hand-out sheet that also has a correction. Few may care, but some might. 

And yes, let your advisor know. Don't worry too much. Mistakes happen.