In the Hoyle canasta rules under the heading How the Game is Played, "You may be able to undo melds if you need to; see the in-game help for details." Where is the in-game help? When/how can a player undo melds?
Talking about in-game help sounds like it's describing a computer implementation of the game, but I'm not sure what implementation it's referring to.
That being said, in real-life play, it's possible to make a mistake in calculating the total point value of a meld. You might think you can meld, so you take the top card of the discard pile and then add other cards from your hand to it. After doing this, if you notice that you actually do not have enough points to meet your current minimum meld, then you must undo your meld, taking your cards back into your hand and putting the card from the discard pile back.