How to read and update a specific field data

I'm doing a small pos system and i want to update the database when I add quantity to an existing item. To do that i need to read the current quantity of that item and then add the new quantity to that value and save data. How can I do that?

this is by sample db:-

Items -LcvTOFQvltop-X8ktBV itemCode:"sc001" itemName:"Nokia 1100" itemPrice:"3500.00" quantity:"7"

for example if I want to add 3 more of this item to the stock I want to update the quantity as 10