What should you do if your date of birth is wrong on your Kukkiwon certificate and card?

I'd discuss it with my teacher first. It shouldn't make a major difference in most cases, unless the age is off by more than a year or two. And you can always contact the Kukkiwon and make arrangements to have it re-issued, but the cost and time may not be worth the effort.

I would wait until my next test and correct the information then.

[Supervisor's note: this question was edited to remove "WTF" since the World Taekwondo Federation is the sport governing body, and has nothing to do with rank certificates.]