On reflection, I don't think the answer I flagged was VLQ. Not HQ by any means, but I was too harsh. I'd like to reverse the flag.
As medica has commented, you can't. Flags once raised cannot be lowered.
Many flags will simply age away if nothing is done. Low-quality flags put the post in a review queue for others to look at, and if nothing else happens then the flag dies.
However, it used one of your flags for that day, it marked the post as flagged, and it caused others to review it where it may not have needed it. Please be careful about what you flag!
That said, you felt at the time that the post was low-quality. Fine. Others may have felt the same; as you don't link to the post in question, it's not possible to check.
Since August 2016, you can retract flags on questions and answers, but not on comments. To retract a flag, click the flag link again and the button should become “Retract Flag”.
The corresponding section of the Help Center has been changed accordingly:
If, while your flag is still waiting to be handled, you re-open the flag dialog, you'll be presented with the option to retract your pending flag:
You cannot raise the same type of flag twice on a post: for example, if you retract a "spam" flag, you won't be able to raise a spam flag or an abusive flag in the future.
Retract flags only when you're sure there's no need for the post to be flagged, or you become convinced that a more serious type of flag is needed.