Open the topic from Topics and make your changes - wording, questions, whatever needs fixing. Edited content is re-screened for safety before it reaches your child, the same as when it was first published, so there may be a short moment before your change takes effect.
What counts against your limit
Only creating a new topic counts against your account's monthly allowance. Editing or simply keeping a topic you've already made costs nothing further, however often you come back to refine it.
Hiding a topic from one child
If a topic isn't right for one particular child - too easy, not their thing, or you'd rather they focus elsewhere for now - you don't need to touch the topic itself. Go to that child's profile and use
to hide it just for them. It disappears from their hub and can be brought back at any time, and it has no effect on any other child who can still see it.
Retiring a topic for good
To take a topic down entirely, open it from Topics and select Take it down. It leaves every child's hub immediately; nothing about a child's existing results for it is lost, since those stay recorded on their history regardless.
A note on the leaderboard
Topics you build yourself are family content, not the shared catalogue - so they're never eligible for the public leaderboard, whatever a child's own leaderboard switch is set to.