When you update the files on a product, you may want to send it out to all customers who previously purchased it. This is easy to do with FetchApp!
Go to the product you wish to update, and add/remove any files that you would like.
Once this is done, click "update buyers" on the righthand menu:
You'll see a dialog that confirms whether you'd like to update all customers who previously purchased the product. Click "send update":
Once you do this, FetchApp will start a queue to send all of your existing customers who purchased that product a followup email. The contents of this email are located in the "Product Update Template" in your email notifications section of your FetchApp account.
Please note that the update buyers feature can take some time to fully deliver, depending on how many files are attached to the product, and how many customers purchased it previously.