I'm not too familiar with the particular solution recipe... I assume you are trying to increment this value by 1. Most likely it has you create a recordset to get the current value of the number of downloads from that field in the database, and so then you would set its value to the current amount you have looked up in a recordset + 1.
If you can see how to set it to the current amount, then you should be able to set it to the current amount + 1.