Get User's selected records and update values

Hi,
Here we will see that how to get (fetch) user's selected records only and apply update method on selected records only (i.e. there are thousands records and user wants to update value only in 10 then user will select 10- records and click on the action button then action should be performed).
Scenario- 1: 

• Create "Button", add it into related form then expand that Button, add Clicked() method and add logic.
• Here CustPackingSlipTrans is the data source of that form where this clicked method is mapped

// <purpose> to update "boolean" value on the basis of user's selected records
void clicked()
{
    CustPackingSlipTrans CustPackingSlipTransLoc,CustPackingSlipTransLocal;
    super();
    //recordcount = ReservationCancellation_ds.recordsMarked().lastIndex();
    CustPackingSlipTransLoc = CustPackingSlipTrans_ds.getFirst(1);
    while(CustPackingSlipTransLoc)
    {
        ttsBegin;
        //CustPackingSlipTransLoc.MSASNExport = NoYes::No;
        //CustPackingSlipTransLoc.update();
        update_recordSet CustPackingSlipTransLocal
            setting MSASNExport = NoYes::No
            where CustPackingSlipTransLocal.RecId == CustPackingSlipTransLoc.RecId;
        ttsCommit;
        CustPackingSlipTransLoc = CustPackingSlipTrans_ds.getNext();
    }
    //info(CustPackingSlipTransLoc.SalesId);
    CustPackingSlipTrans_DS.refresh();
    CustPackingSlipTrans_DS.research();
}

Happy DAXing
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