There is no way for a community member (or a moderator) to edit a flag once it has been submitted. A moderator (or a 10k user for some kinds of flags) has to handle the flag.
This handling does not have to be negative for the submitter or the flagged-answer-poster. If a moderator feels that the post does not deserve the flag, he can just dismiss it (so there should be no penalty for the posters of good answers). Independently of this, the moderator can indicate that the flag was helpful or not. I admit that if a flag seems completely frivolous, I am likely to mark it as unhelpful. But if the post is indeed low-quality (even not as low as to warrant deletion), I would probably mark it as helpful even if I am not aware that the flag was a mistake. And in the rare case that you flag a perfectly good answer by mistake and get an "unhelpful" flag in your story, this doesn't matter much. Helpful/unhelpful flags in somebody's profile are useful for recognizing who has a tendency to cry wolf. A single unhelpful flag among multiple helpful ones is not significant.
Of course, you have some incentive to inform the mods, because when we are aware that it was a honest mistake, we are almost sure to mark the flag as "helpful". As you have no automated way to do so, chat or meta are good possibilities. A moderator who has recently been in chat is pingable, so they will see a notification when they log in on the site, even if they are not online at this moment. Meta questions also create a notification for moderators. Chances are good that we go through our personal notifications list before we go through the flags, so it is almost sure that we will notice your request before we have had the chance to mark your flag as unhelpful.