Changing Slider thumb in SwiftUI

You can do this with SwiftUI-Introspect.

You are basically just accessing the underlying UISlider from SwiftUI's Slider, and then setting it there. This is much easier than creating a custom slider or making a representable.

Code:

struct ContentView: View {
    @State private var value: Double = 0

    var body: some View {
        Slider(value: $value)
            .introspectSlider { slider in
                slider.setThumbImage(UIImage(named: "Play_black"), for: .normal)
            }
    }
}

Result (temporary image):

You can do this using UIKit's appearance in SwiftUI.

Example:

struct CustomSlider : View {
    @State private var value : Double = 0
    init() {
        let thumbImage = UIImage(systemName: "circle.fill")
        UISlider.appearance().setThumbImage(thumbImage, for: .normal)
    }
    
    var body: some View {
        Slider(value: $value)
    }
}

You can't simply modify the thumb of a Slider in SwiftUI. But, you can create a custom slider though.

Here is an example: https://swiftuirecipes.com/blog/custom-slider-in-swiftui