To upload a revised file for your product, you can log into Seller Admin and click View/Edit/Delete Products, select the relevant product and click Edit, then click the Overwrite Product File button (or Replace Product File in our new Admin preview). That will open a file-uploader window where you can Browse to the new file and Upload it in place of the last file you'd uploaded to that product.
The only product settings which affect our button code would be the Item Number, Minimum or Suggested Price (for products that Let Buyer Edit Price), or Variants/Variations settings (for products that use those options).
As long as you only edit an existing product's settings and don't touch those settings, you should not need to replace any button codes already in your site to sell that product, which will automatically pick up any other changes.
