To update the user score, you can use editScoreValueById function available in the Leaderboard service. Logic will remain same like getting the last score, add the new score in last one saved and update with the new score.
Regarding your second query, I would suggest you create multiple games inside AppHQ Management Console for daily leaderboard. And at the time of saving user score, make two API calls. In which one will save user score on a daily level leaderboard and second will save entry on a global leaderboard.
Using the above approach, will not be required you to make all users score to zero, You can directly go and delete the game to make the remove all the entries of the users.
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