How to create action logic for "updating a specific field in already created record"?

I have a button on screen which adds to quantity in my shopping cart. When a user uses that button only the quantity field is supposed to be updated. The record in that entity is created by a separate client action. Please have a look at the attachments to gain a better understanding of my question.

The attachments show the on screen buttons and the database entity which is to be updated. 

The question statement is " How do i create the action which adds/subtracts the quantity in the database entity?"

Hi Mujtaba,

By default, newly created Entities are configured to only update attributes that where modified, so simply using the UpdateEntity entity action would have the desired behaviour.

If you don't trust this, you can always define an UPDATE SQL statement that only updates that particular attribute, but I'd advise against that if you're not comfortable with the platform and SQL.