Hello there!
As of now, the UploadNewEntry could also be used to alter the username of the player. But since you are curious, here is an example:
LeaderboardCreator.UpdateEntryUsername(YOUR_PUBLIC_KEY, "New username", callback: isSuccessful =>
{
if (isSuccessful)
Debug.Log("Username updated successfully");
else
Debug.LogError("Username update failed");
}, errorCallback: errorMessage =>
{
Debug.LogError(errorMessage);
});