Well, this will work a little differently depending on whether you're editing a preset list that is saved locally on your iPhone or the preset list that is on your stompbox. In the first screen for preset lists, what's selected in the source column determines which one of those you're looking at. I'm going to describe the case of editing a local list and transferring it to a H9, and hopefully, that will help you even if that's not exactly what you're doing.
When you're in the lists screen and looking at the representation of the preset list with 99 buttons, i.e. one button for every preset, to assign a preset to a position that already has a preset, you need to click on the button twice; the first time you click on the button it will load the preset and when you click on the button again, it will take you to the preset selector screen. At the top of the 99 button screen is a save button which will save your changes to the list locally on your iPhone. To transfer your edited preset list to an H9, you need to click the back button to go to the opening preset list screen which shows an overview of all of the lists. Clicking on the "To Pedal" button on the top of the screen will send the preset list over to your connected H9.
