If you got less than 3 Stars after completing the Daily Challenge, you can retry it by spending Coins. Retrying can only happen during the regular time frame for that Challenge.
Retrying the stage will get you a completely new set of letters and Required Words.
After completing the Challenge, you will be able to collect another Coin reward. However, you will only keep the best Star score. (For example: If you get 2 of 3 Stars on a Challenge, retry it, and get 1 Star on your second try, it will still show up as having 2 of 3 Stars.)
Retrying the stage will get you a completely new set of letters and Required Words.
After completing the Challenge, you will be able to collect another Coin reward. However, you will only keep the best Star score. (For example: If you get 2 of 3 Stars on a Challenge, retry it, and get 1 Star on your second try, it will still show up as having 2 of 3 Stars.)