You can't "take back" a flag.
It is a much requested feature, but it has never been implemented.
After I've dioscovereddiscovered that flagging is unreversable in all cases (and that includes when the flagged text has been edited), I've been much more reluctant to flag. I flag spam and new questions posted as answers, as those are usually obvious, but I never flag anything that requires a value call, or where there is a remote possibility that editing by the OP or the community may change things.
While some say it doesn't matter, I just don't like the aesthetics of having declined or disputed flags listed on my profile page, so I rarely use the flag.
If my reaction is typical, it may be that this is the intended behaviour. I.e. that the designers of this site want the threshold for flagging to be high, so that only clear cut transgressions are flagged. If it is inteded to condition behaviour in this way, there is probably no point in requesting this feature to be added.