Is there a way to update rows based on current value. So instead of querying current value, you can say quantity + 10 for an update? I know Postgres supports it natively, does the API have a way of doing it?

yes I think something like this should work:

sql
UPDATE yourtable SET yourvalue = CASE
   WHEN yourvalue = 5 THEN yourvalue + 5
   WHEN yourvalue = 10 THEN yourvalue + 10
END

yes, on the sql side there's no issue