Assuming that you mean the files for a dynamic product, you would edit the product in uStore admin, and find the account, campaign and doucument name listed in the document template section. You can then use that information to go to your uProduce dashboard, and locate the document in the account/campaign specified. Or, use uCreate Print connectivity in InDesign to open the relevant document.
If you mean some other product type (static, upload, composite, etc) then it is best to access the files via the orders section of the back office. Depending on which queue the order is in, you have a download link there in the back office.
You should not go changing files in the file system of the server.