Wrong internship dates in my CV for graduate application
Correct the dates on your CV, then check the admissions website to see if there is any information about submitting corrections or amendments to your submitted application.  If there is not, email the corrected CV to the admissions office or department with a very brief line, along the lines of "Hello, I submitted my CV with my applications but later noticed I put the wrong year on one of my internships.  I've corrected it in this version.  Thank you for your time."   Include any relevant reference information to your application.

I honestly wouldn't expect it to get replied to, or possibly even noticed - admissions offices are very busy this time of year.  But this way if it does come up you can refer to your earlier email with the corrected version.

I also wouldn't fret about it over much.  It's remotely possible it could hurt you against an otherwise identically-qualified candidate who didn't make mistakes on their paperwork, assuming anyone even notices the discrepancy.  But even that seems like a very small detail against a good candidate.  The other thing I would suggest is that you proof read the whole thing carefully before you submit it with this correction; if you didn't proof read it well the first time and missed this mistake, it's possible you missed others too.

Good luck!