Programmatically change to another tab in SwiftUI

You just need to update a @State variable responsible for the selection. But if you want to do it from a child View you can pass it as a @Binding variable:

struct ContentView: View {
    @State private var tabSelection = 1
    
    var body: some View {
        TabView(selection: $tabSelection) {
            FirstView(tabSelection: $tabSelection)
                .tabItem {
                    Text("Tab 1")
                }
                .tag(1)
            Text("tab 2")
                .tabItem {
                    Text("Tab 2")
                }
                .tag(2)
        }
    }
}

struct FirstView: View {
    @Binding var tabSelection: Int
    var body: some View {
        Button(action: {
            self.tabSelection = 2
        }) {
            Text("Change to tab 2")
        }
    }
}