I have flagged an answer (PDF conversion on website using ASP.NET platform) by mistake. Is there any option to un-flag it?
According to this feature request on Meta Stack Exchange, there isn't any way to: cancel misclicked flags.
I wouldn't worry about it though. Moderators evaluate each flag, and in doing so will notice things like in this case, that a recent answer to the same question was also flagged (and consequently deleted).
You always have the option of leaving a comment that can be removed by a moderator while reviewing it too.