Instead of passing both the Activities class and the individual Activity, you should be passing in a Binding to the Activity so that you can update it in your detail view.

Here's a very simplified example to demonstrate:

import SwiftUI

struct Activity: Identifiable {
    let id = UUID()
    let name: String
    var numberOfTimes: Int = 0
}

class Activities: ObservableObject {
    @Published var activities: [Activity]

    init() {
        activities = [
            Activity(name: "Jogging"),
            Activity(name: "Reading"),
            Activity(name: "Petting the cat"),
        ]
    }
}

struct ContentView: View {
    @StateObject var activities = Activities()

    var body: some View {
        ActivitiesListView(activities: activities)
    }
}

struct ActivitiesListView: View {
    @ObservedObject var activities: Activities

    var body: some View {
        NavigationView {
            List {
                ForEach($activities.activities) { $activity in
                    NavigationLink(destination: ActivityDetailView(activity: $activity)) {
                        HStack {
                            Text(activity.name)
                            Spacer()
                            Text("\(activity.numberOfTimes)")
                        }
                    }
                }
            }
        }
    }
}

struct ActivityDetailView: View {
    @Binding var activity: Activity

    var body: some View {
        VStack(spacing: 40) {
            Text(activity.name)
                .font(.title)
            Text("\(activity.numberOfTimes)")
                .font(.subheadline)
            Button("Increment") {
                activity.numberOfTimes += 1
            }
        }
    }
}

This syntax is available starting in Swift 5.5 and iOS15 but it's backwards compatible to iOS13 as long as you are compiling with Xcode 13.