Updating the items in the bundle once completed is a common thing that could happen when customers are purchasing bundles. Unfortunately, once you complete the builder process and the bundle is already created you can't go back to edit it anymore. This is just how the app was built, if you go back to the previous page by clicking the back button on your browser, it will only reload the builder URL and will restart the process for a new bundle.
If changes need to be made to the cart, you'll have to remove it from the cart and restart the process or you also can add a custom script to the cart page, something like “Click here to make another builder / start over,” to provide a link for customers to click to start creating a new builder.
We are still continuously improving the app and this is definitely something that we would consider for future development.