How to do I change the background of the app?

Do you want to change the color ?

Simplest is to use ZStack

struct ContentView: View {
    var body: some View {
        ZStack {
            Color.purple
                .ignoresSafeArea()
            VStack {
                Image(systemName: "globe")
                    .imageScale(.large)
                    .foregroundColor(.accentColor)
                Text("Hello, world!")
            }
        }
    }
}

You can use a gradient, an image instead of plain color…

Get details here:

https://sarunw.com/posts/how-to-set-screen-background-color-in-swiftui/