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I don't know for sure whether updating your oversight would adversely affect your chances of getting into the FBI. But I suggest that you give serious consideration to withdrawing your application altogether and seeking employment elsewhere -- not because of your oversight regarding your most recent marijuana usage, but because of the high risk of a false positive on the polygraph and the serious consequences thereof.
About 50% of those who pass Phase 2 are being branded as liars by the polygraph. Given what is known about the accuracy of the polygraph -- the National Academy of Sciences has confirmed that it's junk science -- it is clear that many truthful applicants are being falsely branded as liars.
FBI polygraph results are entered into applicants' FBI HQ file. This is a permanent record that will follow you the rest of your life. Having a failed FBI polygraph on your record will make getting employment with any federal agency that requires a polygraph (or even just a security clearance without the polygraph) more difficult.
I was hoping someone could help with a situation I am in. I passed phase 1 and am scheduled for Phase 2. While filling out FD 140 and during phase 1 exam I signed that I was within the 3 year marijuana usage. Recently I remembered that my most recent time was in July of 2001. This will be 3 years this month. The problem is that I already signed papers the drug report. Should I contact my FBI recruiter and update this oversight, or will this destroy my chances of getting into the FBI. I don't want anything to cause me to fail the poly, and I don't want anything to keep me from thE FBI position. Thanks- claridge