Ah yeah - I forgot about the icons etc - it's somethig I had to work out for myself also - now you know what to do it will become second nature.
Going forward, rather than replacing the icons and launch screens each time you update your app, simply go into the folder that Construct exports (YOURAPPNAME.ios.project) and ther'll be a folder with the same name as your app - go into this folder and just replace the graphics in:
- Images.xcassets/AppIcon.appiconset - your app icons
- Images.xcassets/LaunchImage.launchimage - your launch images
You can also copy your existing Images.xcassets folder and overwrite any subsequent updates with that folder. The images will auto update once you then open Xcode.
To create the graphics themselves I'd suggest using a graphics application that can batch process single images into multiple files - for this I use Sketch App (https://www.sketchapp.com/)