Insert/update Record

I have to compare 2 tables: orderA and orderB, if the record exists in both table orderB and orderA, just do a update to the quantity field in orderB (add orderA.quantity to orderB.quantity). Otherwise a new record has to be insert into orderB.

Do I have to loop through the recordset of orderB, then use select statement to see whether the record exists in orderA or not, and determine whether to update/insert record? I am using Access database. I just afraid that looping through each record in recordset (maybe >=30 record each time) would make the transaction getting slow (or even hang??). Can I do this in one SQL statement?