Hur minskar man ett tal i ett databasfält?

$w('#engBuy').onClick((event) => {
     if (itemId.tokens <= 10) {
         console.log('To few tokens for purchase');
  } else {

    wixData.get('Members', itemId)
        .then((itemId) => {
        itemId.tokens - 10;

    return wixData.update('Members', itemId);
})
        dataset.save()
        .then((updatedItem) => {
            console.log('Item updated:', updatedItem);
         $w('#engSuccess').show();
    })
   .catch((error) => {
            console.error('Error updating item:', error);
  })
}

const dataset = $w('#dataset1');
          dataset.onReady(() => {
    const currentItem = dataset.getCurrentItem();
      if (currentItem) {
    const itemId = currentItem._id;

         $w('#engBuy').onClick((event) => {
    let isChecked = $w("#checkbox1").checked;
    if(isChecked === undefined) {
       console.log("is not checked");
  } else {

    wixData.get('Members', itemId)
        .then((item) => {
        item.inclusiveRole = true;

    return wixData.update('Members', item);
})
         $w('#dataset1').save()
        .then((updatedItem) => {
            console.log('Item updated:', updatedItem);
         $w('#engSuccess').show();
         $w('#engSuccess').text = '"All inclusive" is added to your account!';
    })
   .catch((error) => {
            console.error('Error updating item:', error);
     if ($w("#engSuccess").show())
         setTimeout(function() {
         $w("#engSuccess").hide();
    }, 6000);
   })
  }
})