Assuming your id is increasing ascending, you could use something like the following (syntax isn't correct, you'll have to use your own fields and your library reference in the from):
//Load Sales QVD to get the latest Order date
ORDER By Order_ID ASC;
DROP Table Sales;
//Using -1 for the Row because it is the last record
//Parameters: Peek(FieldName, Record, Table);
Let vLastID = Num(Peek('Order_ID', -1, 'Sort_Sales'));
DROP Table Sort_Sales;
This will give you the latest Order_ID in the variable vLastID.
Now load the records from your source that are higher than vLastID (your increments).
Concatenate your Sales.qvd with that.
Save the new Table as Sales.qvd.
What Ali means is - how do you know the record was updated?
With no "date updated" field how (in pseudo code / English terms) do you know the field has been updated?
If you can't explain that or there is no logic other than the fields gets updated then implementing incremental load is probably not a solution for your problem.
Unless of course you know which ID's were updated (via an Audit table or something similar) in which case you'll:
Load the records from your saved QVD, excluding those ID's.
Add the following:
Load those ID's from source and concatenate them as above to the QVD before saving the new one.