1. I have a UiKit-based app that is currently on the app store.
2. I have created an app with the same functionality with SwiftUI in a new project file.
3. Is there a way to use the new SwiftUI file in the UiKit-based app project file? Do I just need to delete all the existing UiKit code?

I don't want to release a new app, I want a way to update the app.

As long as you use the same app identifier you should be able to upload it to the app store and overwrite the old app.

As you talk about a file. Have you only changed one file to SwiftUI or the whole app?

I created a new project file and implemented all the UI and functionality inside with SwiftUI. I want to apply all of this to my existing UiKit project file to switch everything from UiKit to SwiftUI and delete everything related to UiKit.

When you use the same Bundle Identifier and a build number higher than your existing app it should be possible to upload your new project to the app store under your old app. Check for version number as well.

I understand you to be saying that as long as I use the same bundle identifier as the existing app and a higher build number than the existing app build, I can update from the new project file instead of the old one, is that correct?

When you go on the Apple connect.

1. Add a new version.
2. Change any metadata(Description etc)as well as "What New"
3. Change screenshots(as it proberly look different)
4. Upload the build (making sure that it has the same Bundle ID) and should be same version.
5. Submit for review.
6. If accepted then anyone who has the pervious version will get updated with the new version (SwiftUI)

great! this is exactly what I need in the near future!