https://www.wix.com/corvid/reference/wix-data.html#update
The update() function compares the _id property of the specified item with the _id property values of the items in the specified collection. If an item in the collection has that _id value, update replaced the item’s property values with the ones in the specified item. If the existing item had properties with values and those properties were not included in the specified item, the values in those properties are lost. The item’s _updatedDate property is also updated to the current date.
You might be better using getCurrentItem and then setFieldValue or setFieldValues.
https://www.wix.com/corvid/reference/wix-dataset.Dataset.html#getCurrentItem
https://www.wix.com/corvid/reference/wix-dataset.Dataset.html#setFieldValue
https://www.wix.com/corvid/reference/wix-dataset.Dataset.html#setFieldValues