question

It could be better if you tell us which part you do not understand so that we can offer you some explanations.

According to our documentation Using Resettable Statistics and Leaderboards, to update statistics, you can use UpdatePlayerStatistics API calls

The following is an example.

// first you need to login, I will skip this part
// Then, Define the StatisticUpdate list
List<StatisticUpdate> myUpdate = new List<StatisticUpdate> 
{ 
    new StatisticUpdate 
    {
        StatisticName="your statistic name", Value = 123
    }
};

PlayFabClientAPI.UpdatePlayerStatistics(
    new UpdatePlayerStatisticsRequest
    {
        // you can see in the API document the UpdatePlayerStatisticsRequest needs List<StatisticUpdate> variable
        Statistics = myUpdate
    },
    onSuccessCallback => { },
    onFailureCallback => { }
);

Please see details about UpdatePlayerStatisticsRequest here and StatisticUpdate here.