Updating @Published variable of an ObservableObject inside child view

Try embedding your code in DispatchQueue.main.async like this:

func login(){    DispatchQueue.main.async {        //update UserAuth().isLoggedIn to TRUE    }}

One possibility would be to insert the UserAuth object from the ContentView into the subviews as an EnvironmentObject.

struct ContentView: View {    @ObservedObject var userAuth = UserAuth()    var body: some View {        Group {            if userAuth.isLoggedIn {                MainView()            } else {                AccountView()                .environmentObject(userAuth)            }        }    }}

Now userAuth is accessible in AccountView and all its subviews (e.g. LoginView):

struct LoginView: View {    // Access to the environment object    @EnvironmentObject private var userAuth: UserAuth    var body: some View {        VStack {            Button(action: {                return self.login()            }) {                Text("Log in")                    .padding()                    .foregroundColor(Color.white)                    .background(Color.green)                    .cornerRadius(10)            }        }    }    func login() {        // Update the value on the main thread        DispatchQueue.main.async {            self.userAuth.isLoggedIn = true        }    }}

It may be necessary to insert the EnvironmentObject into the LoginView manually. You can do this by accessing it in the AccountView and inserting it in the LoginView:

struct AccountView: View {    @State private var toggleSheet = false    // Access the value, that was inserted in `ContentView`    @EnvironmentObject private var userAuth: UserAuth    var body: some View {        VStack {            Spacer()            Button(action: {                self.toggleSheet.toggle()            }){                Text("Toggle Modal")                    .padding()                    .foregroundColor(Color.white)                    .background(Color.blue)                    .cornerRadius(10)            }            .sheet(isPresented: self.$toggleSheet){                LoginView()                // Insert UserAuth object again                .environmentObject(self.userAuth)            }            Spacer()        }    }}

Further Reading
WWDC 2019 Video | Hacking with Swift Example